Steven Tyler and Vince Vaughn stepped out separately at Sunset Marquis's Bar 1200 in West Hollywood on Monday night.
Both stars arrived around 10 p.m. – and didn't seem to see each another, an onlooker tells PEOPLE.
"Steven was there with a group of guys and girls," the source adds. "There was a birthday going on. He was posing with his friends for photos. They were laughing and having a great time."
Meanwhile, Vaughn kept a low profile with a few male friends.
"They seemed like just ordinary guys going out to catch up together and share a few laughs," the source says.
Wearing baggy jeans and running shoes, Vaughn was "super chatty and polite" to whomever walked by.
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