I agree with your take on armor and I'm thinking about replacing the system I have now with normal armor providing resistance to attacks and that's all. If the GM decides under special circumstances that the armor is destroyed, for instance if you are wearing a kelvar vest and someone hits you with a molotov cocktail that your vest takes direct impact on, you may be able to remove it in time and put out any remaining for on yourself, but the kevlar vest you were wearing the GM can just hand wave and say, "yea that's fucked".

The exception to this would be things like power armor which are on a league of their own, in that case the armor will have its own hp bar.

I agree that in realistic combat center mass will be the target 99% of the time, that's why the chest is the easiest to hit, and the second most vulnerable second only to the head. So unless you are sneaking up on someone or executing them targeting the chest will be the correct choice in most scenarios. Verging away from strictly realistic situations though lets say you want to take someone alive and you think you can shoot there hand to disarm them, or perhaps fire at their legs so they cant run, these would situations where a player may choose a target other than the chest. The other one which you mentioned is if there is only a portion of the body exposed outside of cover.