I can testify from personal experience that one 9mm round, COM to a subject wearing a vest, is a SERIOUS blow. It may be less than likely to kill. But it may certainly knock the wind out, render an arm temporarily useless and/or make the subject feel as if he's about to die. It would certainly give the shooter a "time to run away" advantage, or if the situation dictates, take more careful aim.
Sgt Lumpy - n0eq