

Melee weapons can definitely get a way higher sneak attack multiplier. There's a lot of debate over whether or not criticals can occur outside of VATS in Fallout 4, and debate over whether or not sneak criticals are still a thing. In Fallout 3 and New Vegas there were specific sneak criticals and criticals could also happen outside of VATS. There is plenty that increases your ability to sneak, but I believe this is only relevant to enemies actually detecting you in the first place.

I may be wrong, but I don't think there's any armor that actually increases sneak attack bonuses.
