A source told People magazine: "Vanessa was bopping around with her arms up while Zac had his hands all over her. Vanessa had a smile on from ear to ear and the two were inseparable after that.”
Following their public display of affection on the dance floor, Zac, 23, and 22-year-old Vanessa then retreated to a table – and grew even more affectionate.
The insider explained: "Zac sat in between her legs and she wrapped her arms around him. They were making out on and off for the last hour or so until closing. They acted like there was no one else in the room."
The former couple narrowly escaped being pictured together but as the ‘17 Again’ hunk leaned in for a kiss, Vanessa pushed him away after spotting a fellow reveler with a camera phone.
However, a source said: "They were kissing on the lips and seemed to be enjoying their night. They seem to be back together."
When news of Zac and Vanessa’s break-up was announced last month, pals insisted the separation was mutual and they were still “good friends”.
The pair met on the set of ‘High School Musical’ in 2005.