In a gunfight, first and foremost, you are going for Stopping power, not kills per se... Unless your walking around wearing b-52 armor plating (though walking might be a bit hard with that on...) getting hit in the chest, even with a bullet proof vest on is going to knock your ass flat and out of commision for a good bit. Yes the vest may stop the bullet from hitting your flesh, but the power of the impact is still going to be distributed onto your chest mainly. Even such a small object, when travelling at such great speeds hits you, it's going to have a LOT of force behind it.... picture maybe a steroid'd up baseball player hitting you full power in the chest with a bat... it's going to knock the wind out of you good.
In all my range testing, for a 9mm and the rifle, I've rated expert, (top navy marksman) and they still always say go for the chest because you want to disable them first, then worry about killing if needed... disabled means non combantant and they won't fire back.. and since hitting the chest is easier and will still knock em out of the fight for a good bit it's the better bet.