Vicious is a mean-looking super mutant who none the less has a softer side and appreciates a good joke. He is in charge of the ops center, ensuring that everything proceeds smoothly and that only select people can see the Master. He is a heavy smoker, leaving piles of cigarette butts in the storage room he smokes in.
Vicious will attack the player if their weapon is drawn.
Vicious will question the player what they're doing at the lowest level of the Vault. Since he only does that if interacted with, it's entirely possible to avoid him entirely. He will either suspect something is amiss or clear the player through.