I was taught in my training classes a couple points I'd like to share.
First, gun fights last seconds, not minutes or hours. Hunting a head takes valuable time, time you are not likely to be afforded during an attack.
Second, should you encounter a shooter that you expect is wearing body armor, engage with multiple shots, center of mass, while moving off the line of force. Armor or not, he's gonna feel it. This buys you precious time. Time you can spend hunting the head.
The head of the company I've trained through had some examples of body armor shots and it was clear that even wearing level III armor, you'd sure as hell feel a 230 grain Hydro Shock.