He is not working for him, just covering his friend. Don't you have a friend IRL, who you took care no matter what?
Nick was looking for laptop in library because he suspect Vinny did it, not cuz he knows it. His room wasn't trashed cuz Vinny just couldn't do it to his friend. We know that Nick meets Vinny from time to time, Vinny left the DIKs after losing the fight for VP with Tommy. Vinny is drug dealer just like Quinn and Quinn trust him.
There is many details that lead to conclusion that Vinny did the Cathy cluck (all DIKs have access to Rooster account before cluck. Cathy's photo was taken in jocks gym for sure, pretty sure Dawe took it cuz he hates Cathy), trashed the mansion after jocks (jocks definitely broke the windows - you can even see the dildo, that Dawe was holding, in library room on the floor). Remember hand writing in Rusty's room? Vinny definitely did it just like he did in library while pledging.
What for he did it? That's the question holding for now.