The 22-year-old actor - who has been dating the brunette beauty since they met on the set of `High School Musical` in 2005 - was watching the actress strut her stuff in musical `Rent` at The Hollywood Bowl when he got up from his seat to tip her for her sexy striptease during `Out Tonight`.
Vanessa - who is starring as HIV-infected stripper Mimi in the popular stage production - grabbed the cash from her boyfriend and seductively stuffed it into her top.
An onlooker said: "Everyone was surprised when Zac got out of his seat to give Vanessa the money. It was all very good humoured although she was probably shocked to receive a bill from her boyfriend during the middle of the show."
Perhaps the actor picked up tips on how to deal with raunchy dancers from his recent visit to New York strip joint Flashdancers Gentlemen`s Club with fellow `High School Musical` star Corbin Bleu and another male friend.
He is believed to have spent $2,000 during the evening out and requested dances from three sexy brunettes.
Speaking about the evening, Zac explained: "The hardest part was calling my girlfriend beforehand. How do you start that conversation without feeling like a total a*shole? It was hard. But she was fine with it.
"I`m not really the type of guy that does that kind of thing often. I envisioned myself sitting on a nice couch, shades on, with T-Pain and Usher, making it rain money - and it just wasn`t like that."