I dont know for sure but I doubt that a slug would penetrate body armor.
Soft body armor is designed specifically for that type of projectile; large, low velocity and large frontal area are what Kevlar webbing is designed to stop.
But the energy that a 12 gauge slug carries would have to be absorbed some way. You may not be killed by the slug penetrating your body but you may be killed by a broken rib penetrating your lung or heart.
Shotgun slugs destroying an engine.
That said, nobody has developed a slug-proof helmet visor yet...
My thoughts, too. At the very least, the wearer would likely be knocked off his feet.