Using Armored Plates
 

How do I use them. All I have is the M200 concealable ballistic vest and the armored inserts dont work. I assume I need a different vest but havent run across one yet.

It should say whether it supports additional plates. I think the very first thing you find, the olive-drab kevlar vest doesn't support them. Sooner or later you'll find Wildcat vests and a heavier Russian vest.

Perhaps the external vests also allow plating, but by the time the enemies spawn with those I've finished the game.