The pair, who announced their split in December, appeared to be more than friends at Friday evening's SHG's Eden Hollywood opening, where they were "constantly touching and kissing each other," an onlooker tells PEOPLE.
After arriving with pal Brittany Snow around midnight, the actors quickly took to the dance floor. "Vanessa was bopping around with her arms up while Zac had his hands all over her," the source says. "Vanessa had a smile on from ear to ear and the two were inseparable after that."
Eventually heading towards a table, the couple proceeded to get cozy – and stay that way for the rest of the evening.
"Zac sat in between her legs and she wrapped her arms around him," the onlooker notes. "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."
Reporting by JENNIFER GARCIA