Simple, shoot them and keep shooting them until you are out of ammo and/or they are no longer a threat.
Humans are not robots, these sorts of people are assuming that lead is going to only be flying one way. Even among professionals, when ranges go two way marksmanship goes to crap. Kevlar only keeps the round from penetrating, it doesn't do anything about Newtons laws of motion. What effect that will have on a crazy person is anyone's guess, but I can't think of anything that would make things worse, they might panic and run away, their careful aiming might give way to spraying.
As the 86 Miami shootout showed, you also have a good chance of hitting them in the arm while aiming center mass, rarely does anyone have armor there, injure their arms and they are going to have a much harder time aiming and reloading.
I don't carry a gun to go looking for trouble, I carry a gun in case trouble finds me.