I'm tempted to throw up a completely inconsequential massive rant post about hippies protesting whatever Government form the perspective of Willow Star Child, blazed activist and vagabond.
It possibly could, honestly. The way Kevlar works it that the fibers catch and bind up as the projectile attempts to pass through the layers, stopping the slugs momentum.
I suppose it depends on the threat level of the armor. Assuming there's a trauma plate and not just a soft armor shell he'd be fine.
In certain situations they could be. It really again depends on the opponent. There is no optimal weapon or configuration, that's why there are so many.