They do Gizmo's dirty work, and basically anything else they want. Vinnie has a deal with Killian Darkwater - the Skulz do not bother Killian and he does not bother them as long as they keep it low-key. The Vault Dweller can put a stop to their activities for good in a quest given by Lars.
The leader of the gang is Vinnie, and other notable members are Victor, Shark, and Sherry. Their name is a pun to the word "skulls."