An eyewitness tells TMZ ... the movie star was spotted at the Barbershop Cuts & Cocktails Wednesday inside The Cosmopolitan, where he joined the resident Tomlinson Brown Band for some sing-along fun.

As you can see ... Vince lets the band do most of the heavy lifting while he joined them to perform Hank Williams Jr.'s "Family Tradition," and Alan Jackson's "Chattahoochee."

Hey, sometimes it's all about enjoying the ride!

Vince was clearly having a great time, and the crowd was loving every bit of his surprise cameo -- so it was definitely good vibes all around.

After he was done onstage, we're told the rest of Vaughn's evening was all about unwinding and enjoying drinks with his group of friends. In other words, the perfect Las Vegas night!