Vests, or plates, don't cover the entire chest cavity. They usually go as high as your jugular notch in the font, top of the shoulder blades in the rear, and halfway up your ribs on the sides. This is so you can still move in them. There are lots of places & angles where you could shoot someone with a vest and still impact the chest. Not to mention bullets don't always travel in straight lines once they impact a dense object like a bone.

