If he isn't wearing a trauma plate (or it goes around it), it would probably produce a whopping deep bruise, maybe knock the wind out of him if in the solar plexus. If it is a Brenneke slug, and maybe some of the sabot types, a bigger bruising.
Some vests are better at dispersing this blunt trauma effect than others without the plate inserts. The build physique of the target would probably affect this too.
Incidently, the Mozambique drill applies to shotguns (and rifles) as well as pistols: Two to center chest; if that fails, the next one to the middle of the nose. In some circumstances I might even modify that to one center chest, and if that fails, the next one in the proboscus.
Just because two slugs or two rifle bullets doesn't "stop" an attacker doesn't neccessarily mean the thug is wearing a vest either. There are instances where the badguy has absorbed several of either and still not been persuaded.
I can attest to the last part of what LAK is saying. I personally use Brenekke MP slugs for deer hunting. They are 100% killers but not right away.
Last year I tagged a big doe right through the shoulders at 90yds, she swapped ends and I got her again through the heart at 95yds. So no front shoulders and no heart but it still went 40yds, uphill in to the most tangled part of Michigan ever trod by man.
To kill, you must hit but to STOP you must BREAK!
The year before I hit a buck at 48 yds in the spine. I had to track him for 36 inches, straight down!
In answer to your question, probably depend on if the guy was commited to getting you. Adrenalin would control whether he goes down or not if there was no penetration or additional trauma.