The idea about the kevlar vest wouldn't make you just able to take more lethal hits and continue fighting, instead the vest would make it possible for you to become wounded on otherwise a lethal hit.
The main reason for the vest to become available in the game is to bring sense to having rare items available.
The current system of dropping your weapon when getting killed + spawning again with that same weapon would make rare weapons work against their idea, as you just end up
creating more of these rare weapons each time you die. The weapon doesn't stay rare very long that way.
Making you not drop your rare weapon doesn't sound good either, why would these weapons be an exception of regular weapons? Also, getting to loot stuff from others players should spice things up a bit.
We have this laid out so that, in this mode, you'd drop all your
physical items when you get killed, your weapons, nades, kits, any other items you may have. When you spawn again, you'll be given a random weapon from a regular weapon set and some amount of nades, bazookas, similar to how it is now. The fact that you find a rare item which you want to keep using forces you to try not getting (completely) killed, and this is where the vest comes into play.
As we like the "roughly one"-hit kill system so much, having a vest would make you wounded instead of killed when you take that lethal hit. Being wounded means that you don't lose all your stuff, but you can't fight either, not very well at least. Someone needs to get there to revive you (-- another player might not do that unless reviving is properly rewarded; the player might want your stuff instead

). Getting killed by friendly grenades + dropping items may need some adjusting, otherwise players might want to kill each other even on the same side to steal items.
Once your vest becomes discarded for taking too many hits, you better find another one soon or stash your rare weapon if you want to keep having it around through battles.
Of course, having rare items for map matches that take usually max an hour wouldn't make much sense, so the player profiles along their items need to be saved on the server so that you can continue with your stuff and unit.