First of all, very few BG's will be wearing body armor. Unless they're impersonating police or raiding your house for copious amounts of cash and/or drugs, most simply don't think that far in advance.
Secondly, most vests do not allow someone to impersonate Superman, where bullets just glance off. Usually the impact force will knock the wind out of someone; disorient them for a moment or two and in some cases even put them on their knees.
Plus, AP pistol ammo is verbotten by federal law,''