A 9mm pistol round has enormous penetrating power at close range, especially if Benny used +P ammunition. Thats the problem with 9mm, it tends to go right Through its target instead of imparting all of its energy Into the target. Thats why alot of peeps like .45 and other large or powerful pistol rounds; they are better at knocking people down and inducing hydrostatic shock than the smaller caliburs.
Fallout universe or not, at < 3 feet those bullets are going into your head.
That it was a 9mm pistol makes it at least plausible that someone could survive the hits depending on where you got shot, how far the bullets penetrated, etc.