I've been a LEO for 15 years, and during that time the only BG's with vests I have come across have been bank robbers and drug dealers. The drug dealers have them because they are subject to being robbed for their drugs or money. A citizen with a CCW permit is most likely to run into trouble with an armed robber, and these guys don't wear vests for a couple of reasons. Your typical robber picks his target of opportunity based on who looks like an easy mark: someone out of place in his environment, not paying attention, drunk and stumbling around, etc. His objective is not to get into a gun battle, but to get money quickly with as little danger to himself as possible. If he is carrying a gun, he won't have spare ammo, a holster, or a vest because if the cops drive by and want to have a closer look at him, he can ditch the gun by tossing it (or running and tossing it), and there is no way to make a good arrest on him. A holster, vest, or spare ammo goes a long way to proving the gun he tossed belongs to him. Although most BG's aren't legal scholars, there are also laws against felons having vests or possesion of ammo, and word of this is starting to spread based on the current Project Safe Neighborhoods initiatives (which enforce felon-in-possession laws.)
Even if a BG is wearing a vest, he is not going to stick around if someone is shooting at him. Bear in mind, these guys know if you get a lucky shot in, they could get hurt or killed and if they go to the hospital with a gunshot wound, the staff will report it to the police.
Changing your ammo to defeat most III or IIIA vests would require you to go to a rifle round, shotgun slug, a specialty round like the armor piercing teflon-coated-brass-bullets (try explaining that one to the good guys), or one of the fast, small caliber rounds like the .22 WMRF or the 7.62 Tokarev. A lot of trouble for no real advantage, IMHO. The most likely scenario is still that the BG sees the gun, hears the bang, and begins to quickly recede into the distance, whether you hit him or not.