so you wanna be a medic who mains the vaccinator
alright but first off you gotta know some basic basic info

a medic is a portable non ammo giving dispenser. He is very vital in making sure teammates stay alive and enemies do not.
let me first off say that right off the bat:
a medic's MAIN PURPOSE is to HEAL NOT FIGHT !!!! Fighting should only be used for self defense, otherwise you deplete the purpose of going medic to begin with.
That being said, it doesn't mean a medic can only heal nonstop, but try to put it above hitting enemies
sometimes its helpful with say the ubersaw for uber build, or stopping a spy/enemy in general for killing off your ally/patient

anyhow, you as a medic have a sworn duty to heal people and more importantly, choose who to heal, overheal, and most of all, WHO TO UBER!

prority varies but it generally should come down to something like:
-top players in most cases are a main priority. even if it is a spy or sniper, healing and overhealing can increase their output 10X folds.
-the weakest player should also be healed not only to stop him from dying and give him an extra boost, but to give you an extra boost in terms of uberchage build as low health players build faster than full health/overhealed

then by class if skill isn't an issue is (based on opinion as well as gameplay not so much fact). This is also in general as a medic when choosing who to heal/uber:
-heavy=benefits most from healing and overhealing as well as ubecharges especially. they are literal walking tanks. That being said also politely ask for their sandviches that they won't need if you are healing them, but you might if you are burning, bleeding, or about to die.
-other medics=benefits the entire team as 2 medics are better than 1 solo, even if that one solo has a quickfix. Plus they may return the favor later on in terms of healing as well.
-soldiers=2nd to benefit from most ubercharges and 3rd in terms of healing priority. Overheal helps a ton as soldiers are prone to rocket jump and dive bomb the enemy. Soldier is the most balanced class in TF2 in terms of class statistics and versatility in weapons. it is one of the easiest classes to pick up on and master with little to no hardships.
-demos=demos are a main source of defense/area denail. One demo, with the proper training, can stop or halt an entire team long enough for back up to arrive or re-enforcements to arrive. prone to close range attacks and long range, they are a good target to keep overhealed, especially if they sticky jump.
-pyros=one of the most underrated and underpowered in terms of base damage classes out there, the pyro can do miracles if helped out. easily the middle ground class in terms of healing, it can go both ways.
-scouts=can benefit alot from overheal, but just heal them every now and then. Scouts are fast enough to run away, get health, steal your healthpacks and ammo, and in general die off quickly once targeted by the enemy. almost never uber scouts as they are too fast to follow (excluding the quick-fix) and die too soon for the charge to do any effect.
-snipers=usually in the back end of the team/map, snipers are not a good class to choose as a patient. healing them every now and then helps but its better to shoot a crossbow bolt at them from a distance (distance increases healing from it) and continue moving towards better patients.
-engineers=not a good class to have as a patient. Don't get me wrong, as an engy main it would be nice to have a medic making sure i don't die while repairing/defending my buildings. But, the medic could be helping someone else better than me. Also engineers have dispensers for health, which sorta depeltes the purpose of you healing them. They usually aren't in any immediate danger if next to a building, more specifically their sentry as it will guard them from harms way.
-spies=the least likely patient you want to have. spies have low health and low damage output when fighting against a group. spies rely on stealth and disguise alot, so make sure you don't accidentally give their position away by healing one. God knows how many times that has happened to me when i got spy and get healed by a friendly medic on the same team. overheal works great to make sure they can tank a little damage, just enough so if they get caught they can retreat and retry again.

ever since the gun mettle update alot of weapons were reworked
most of the "crutch" or overused weapons got nerfed/reworked to be more fair/skill based
alot of underpowered weapons got buffed.

and unforunately the vaccinator got the short end of the stick. It got some tweaks that try to lvl it out, but for the most part, most people see it as underpowered/nerfed.

PRE-GUNMETTLE STATS
Stats:

+50% Übercharge rate

Press your reload key to cycle through resist types.

While healing, provides you and your target with a constant 10% resistance to the selected damage type.

-66% Overheal build rate.

Übercharge provides 75% resistance to a selected damage type. You are healed for 25% of the matched incoming damage on your heal target.

back in the day, the vaccinator's only downside was overheal as well as wasting an uber slot if the uber was incompletely charged when the previous uber was depolyed. This was overcome by simply getting another medic to overheal your patient to full overheal and you continually overhealing them to stand a better fighting chance. The lost uber meter charge wasn't an issue as most of the charges were used in a row anyhow (they had to to be as effective as possible)

Just healing your patient alone gave you instant crit immunity, so bullet resistance healed patients didn't get any crit shots from sniper/spy headshot weapons. This was the same for any crit boosted/random crit boosted weapon such as a crit rocket, sticky,bullets, and fire, more specifically the phlogistinator for pyro.

the other good thing for this weapon was you would get healed a % of whatever damage your patient took, provided that it was the right resistance. Meaning you wouldn't have to fear say a pyro's flame reaching you as even if you were set on fire, your burning patient tanking the main fire damage would heal you back.

POST GUN METTLE
with the new stats

+50% Übercharge rate
Press your reload key to cycle through resist types.
While healing, provides you and your target with a constant 10% resistance to the selected damage type.
You are healed for 10% of the matched incoming damage on your patient.

-66% ÜberCharge rate on Overhealed patients
-66% Overheal build rate.

Übercharge provides a 2.5 second resistance bubble that blocks 75% base damage and 100% crit damage of the selected type to the Medic and Patient.

first off they removed the crit immunity from just healing the patient. So crit damage can now only be repelled by ubercharging the patient.
they added a -66% ubercharge penatly if you are healing an overhealed patient, meaning unlike before, you can't stick to one patient and build uber otherwise you would get slowed down
they also lowered the amount of health you get from the damage your patient tanks siginficantly, although this doesn't pose a real threat so long as you use the correct resistance.

on the plus side
-ubermeter/charge won't get lost if you pop an uber slot prematurely before the next one is filled completely. This is a critical buff since sometimes ubercharge would be lost and you wouldn't get the chance to save you or your patient.
-longer uber duration. This is also good as now you don't have to pop more than 2 ubers when as opposed to 3-4 to get the same results pre-gun mettle.

well first off it's still the same mechanism, you just have to be more careful with it now as no more automatic crit immunity.
the 2nd most important tip is that there is no melee resistance, meaning you are still prone to crits as well as full damage, unless they try to taunt kill you, in which case if you get killed by a taunt:melee, you deserve to die

anyhow onto the resistances:

the vaccinator offers resistance to all except melee damage
they are classified as bullet/projectile, rocket, and fire resistances.
the vaccinator can only protect against 1 type of damage, although you can cycle through them. Ubercharges, however, only protect from the crit immunity and damage type of just 1 type. So, how to prioritze damage layout

the normal 1 on 1 scenario is whatever class you are up against
if there are enemies around them, both visible and unseen/out of range, then the priority goes:
-bullet resistance
-explosion resistance
-fire resistance (always this one to the very lowest priority with 2 exceptions)

bullet resistance
bullet resistance is THE #1 RESISTANCE YOU SHOULD HAVE AT ALL TIMES almost a good 60-80% of damage types are bullet. This includes projectiles such as arrows (if on fire, fire damage is fire damage/afterburn damage), the direct hit from flares (not the afterburn from the fire, thats another type of damage), pomson, rescue range shots (also before you ask yes they CAN be reflected by airblast but lets save that for another guide), baseballs and ornamets, and any other throwables i may have missed that inflict damage such as the scout's meat cleaver

the 3 most powerful bullet based weapons/classes are:
heavy's minigun
engineer's sentry, the rockets are powerful, but most of the firepower comes from the metal shards/metal bullets it shoots.

and
snipers in general as their primaries, aside from the sydney sleeper, all cause crits on headshots. Not only that, but for every sniper you see, there are 2 you don't due to the nature of most servers (this goes for spies as well).

bullet resistance is the resistance you should immediately have when charging or going into an enclosed area, such as around a corner, in a room, behind a door that opens, ect.
You should, however, uber when entering. This is a big precaution step as it can mean the difference between your patient dying or escaping with less than 10% health but still alive. This goes especially well with maps that have a small gap or open range, where skilled snipers can easily shoot you or your patient down.

explosion resistance
probably the above image come to mind when you think of explosions, with good reason. A soldier isn't as much of a threat as a demo man. a soldier needs to appear in front of you to shoot a rocket and do direct damage to maxmize his potential.
This goes though without saying that both are equally as deadly, but demo should take more/top priority over a soldier.
Reason:
STICKIES
stickies, even without crits, do a ton of damage for being so small, but they are strength in numbers.
not only that, but they also have a grenade launcher which, unlike rockets, if it misses it doesn't explode (excluding the loch n load) and can linger around at your feet, which may not be as bad for your patient, but can take a toll on your health as a medic.

as for the soldier all should go well
however they are prone to random crits more than demomen, something to watch out for
an exception to soldier over demo priority would be if they have a spam weapon such as the air strike or beggars bazooka
they are more prone to random crits and can deal a ton of damage /splash damage in just a short amount of time. If you see them have your patient take priority over the demo spams. other than that the resistance goes hand in hand with both classes

the rocket resistance really doesn't do much for sentries, as you should be more concerned for the bullet damage than the rocket damage.

should you chase a demo down, always uber when turning corners. Demos have a tendency to place stickies when retreating while walking backwards

fire resistance
pyro is the most or if not one of the most underpowered class in TF2. Hence fire resistance should always be at the lowest priority. pyros need to get close to do any significant damage with their flamethrower, and flares aren't as much of a threat as they are dodgeable.
there are however some exceptions to this rule:

-random crits
although not likely/rare, flamethrowers and flares do get random crits from time to time

-phlogistinator or phlog for short
this is a game changer for pyro
at the cost of airblast, you get to charge up for crits and a health refill. Any type of fire damage will fill it up, making flares a weapon to be feared/avoided.

if you see a crit boosted phlog pyro or a phlog pyro charging up by taunting, drop everything and fire resistance uber the closest patient nearby. It only takes 1-2 ubers before your patient kills the phlog pyro, but it takes the pyro seconds to decimate your team, your patient, and you yourself.

other than that normally fire resistance healing a burning player will do if there are no pyros to extinguish them on your team (or there are pyros they just don't extinguish anyways)

Random crits
One of the vaccinator's most solid mechanism is 100% crit damage reduction on uber. Making this the ideal weapon to counter anything that gives you a crit boost (excluding melee). Buff banner, Kritzering, GRU activated crits on a heavy who proceeds to pull out his tomislave/minigun in general, crit a cola, and other crit granting mechanisms such as halloween crit trick or treat pumkins, capturing the intelligence on some gamemodes, ect.
if you see any glowing weapons, excluding melee (if they die by it they deserve to die, also nothing you can do. This would be a better job for another weapon such as the crusader's crossbow or another medic), uber the teammate closest to them to not only distract them from the rest of your group, but also to take them out if possible. If not retreat to a distance if your patient is a highly damage prone class like scout sniper or spy.

AWESOME
if you are reading this you actually took the time to better your self knowledge wise and are on your way to improve yourself as a medic and teammate in general

some couple of closing statements before i finish this about the medic as a whole

good weapons to pair with the vaccinator:
-crossbow=the most universal and versatile medic primary, it not only heals teammates, but also damages enemies depending on who you hit. it increases healing/damage as the distances does as well
-overdose=since vaccinators fill up rather quick regardless of who you heal/compared to other mediguns, the overdose is nice if your patient dies (which unless theres more than 1 damage type that shouldn't happen)
-amputator=since the vaccinator relies on damaged teammates for its uber build, it may not seem a good weapon of choice, but riddle me this. You are the only medic on the team. All you have is the vaccinator. instead of switching to the quickfix, just pull this sucker out, taunt, and heal everyone around you. Also exposes spies as they don't get healed or show the aura ring at their feet/around their feet. It can also help you not die as it increases your health regeneration rate by 3 pts, more if you taunt heal. paired with the overdose, it is a good combination to have.
-solem vow=more utility than use, help your patient pick out which low health enemies to pick off first, or if you are a cocky mofo, which high ranked, highly healed/overhealed enemies to pick off to give your team an easier push/defense


some tips for medic plays:

-it should be known but if not, always make sure your patient knows that a vaccinator is not a complete uber. He is only resistant to the damage type you have, and mostly only on ubercharges. i probably should've mentioned this earlier but im giving the benefit of the doubt. However, if you see that your teammates look like they don't know (stereotypical gibus merc set), then tell them before hand.

-when healing it sometimes helps to walk backwards. This can prevent an ambush, or you can stop a spy from backstabbing you or your patient. Your medigun is pliable / flexible enough to heal your patient while walking backwards or turning corners.

-refering back to the previous one, although you are not physically engaging in combat when healing, you can be an extra pair of eyes. Always look around areas as well as above and behind you. These are the places soldiers, scouts, and spies typically choose to flank you.

-keep your distance. enough to heal your patient but also to escape should things get heated/your patient dies. Again, the medigun is very flexible in terms of healing. keep this in mind when turning corners as well.

-never stick to 1 patient, always heal everyone, especially other medics

-never be "that guy" who complains that there are no medics but doesn't wanna play as him.

-going back to the previous one, everyone wants a medic, but no one wants to do the dirty work. Then again if you are reading this you probably want to be a medic regardless, in which case good for you

-ignore people who tell you to go a specific loadout/medigun. As a medic any medigun heals. That is their main strength. just because each has a different uber, doesn't make it any less special in terms of healing. Ask them if they can go medic and if they refuse, tell them to stop complaining. or stop healing them altogether whichever passive agressive technique works for you

and lastly

have fun :)