What happens when a bulletproof vest is dameged?

Answer 1

That depends on the type of body armor, but usually the bullet deforms and is slowed down by the fabric of the body armor and the energy is displaced throughout the vest. The bullet may or may not remain in the fabric of the vest.

Answer 2

depends, there are two scenarios:

scenario one; it's damaged before you get shot, you can die or suffer internal injuries

scenario two; its damaged after you get shot, throw it away. It did its job, and you should be grateful, but it's damaged, so it won't be much help anymore