This is my opinion on how armor should work on a game like DayZ. I will be explaining my opinion through the experience it should have in gameplay, not by simply looking at the numbers. If this post is too long then The first picture i provide should give a good enough overview of what i think, so look at that. Here it is.
For reference, As of 1.12.153904 patch, Damages are as follows: 2 to kill for stab vest, 2 to kill for press vest, 4 to kill for plate (against Winnie, Mosin, Blaze).
In the picture i use the strongest rounds in the game to make my point. Even though the game does not only care about the caliber of the round anymore, this is still roughly what it should be (imo).
So, the game currently has 3 vests that offer protection against firearms: stab vest, press vest and plate carrier. Each of those vests has a specific weight, inventory capacity and protection values. In a game like DayZ, weapons and armor are balanced by the amount spawning and the location where they spawn. Stab vest should be the easiest one to find but offer the least protection, press vest should be medium difficulty to find and medium protection and plate carrier should be hardest to find and offer best protection. The vests don't need to be balanced in any other fashion but that. I am mainly saying this to combat the idea that Stab vest should offer more protection than press vest for the sake of "balance". Balance is in the amount and location spawning. The same way as m4 is clearly a better rifle than MP5 for example, the balance is again in the amount spawning and location.
Now when we talk about balancing the health and shock protection values, it is important to keep in mind how the numbers affect the actual gameplay experience, not simply how they look on the paper. This is why I made the illustration the way that i did. Stab vest is a vest used to protect against a kitchen knife not the strongest rounds in the game. Obviously for some gameplay balancing it can offer some protection against bullets, but not enough to tank a the strongest rounds in the game (762x54/.308). I feel this is a good balance between realism and gameplay balance. Press vest should be a step up from the stab vest and should now protect you from the worst consequence in a game like dayz which is instadeath. Being shot by a high caliber round like 7.62x54/.308 press vest should still keep you alive but leave you in Unconcious state, giving your friend a chance to save you from the follow up shot or atleast have the grass provide you with cover when you fall down. The next one is plate carrier. This is the vest hardest to find in the game and should now be an improvement over the press vest. Therefore, being shot in the chest by the strongest rounds in the game should perhaps make you stumble but not knock you out. Giving you a chance to reposition soon after. Obviously health damage should be different between the press vest and the plate aswell. I feel like with press vest being yellow health and with plate 3/4 white is a good point. I feel like this is a very logical progression system and would make a lot of sense to all players. A lot of this is already like this in the game. Exception being the stab vest protecting against 7.62x54 and .308 under pristine condition. I feel like stab vest should be nerfed to a point that this is no longer the case and also right now it takes 4 shots to the body to kill a player wearing a plate with a mosin/winchest/blaze instead of a more reasonable amount 3. Shock is at a good place when being shot the plate and press vest i feel. (Not the case when getting shot to the legs but this is a different topic). Once to the stab = Dead, Once to the press = uncon, twice to the plate = uncon.
Obviously sniper rifles are not the only weapons in the game. I feel like the damages on guns already follow a very good logical pattern and this should remain the same.
Weapons ranked from strongest to weakest. I know shotguns and 9x39 guns are missing but they are difficult to place here because of the ammo difference and the difference in range so i left them out. But they work well imo.
MOSIN, WINCHESTER, BLAZE
DEAGLE, REPEATER, MAGNUM
To sum it up, here is what i think should be changed:
1. Stab vest should be nerfed so it does not Tank a 7.62x54/.308 rounds from any of the guns that use those rounds.
2. Plate Carrier should be nerfed slightly, so it takes 3 shots to kill with Winchester, Mosin, Blaze (The same or 4 with SVD, FAL). Shock is good against plate carrier, 1 shot to the body shouldn't uncon, 2 shots to the body in a row should.
3. Being shot to the legs with 7.62x54/.308 should break your leg or have very high chance to break but not uncon you straight away.
For the rest of the guns, adjust them using the same pattern as you already have. RIfles with longer barrels dealing more dmg than fully automatic ones etc.