Credit: Ethan Miller/Getty Images; Mathew Imaging/WireImage.com

When Charlie Sheen reportedly pounded back vodka shots in Las Vegas on Monday night, he wasn't alone: The Two and a Half Men star was accompanied by three adult film actresses, TMZ reports -- including Bree Olson.

It's not 23-year-old Olson's first brush with mainstream celebrity, either: TMZ reports that she appeared as a nanny on Keeping Up with the Kardashians.

What's more, the star even has indirect links to Sandra Bullock: Olson made out with Michelle "Bombshell" McGee, Jesse James' mistress, at The Palms Hotel on Sunday.

As for Olson's famous new pal Sheen, witnesses overheard him complaining -- as he downed Grey Goose vodka -- that CBS execs were urging him to go back to rehab.

Sheen ditched his sober coach -- hired after his Oct. 25 freakout inside an NYC hotel room -- just before the holidays.

"The sober coach never went back to work for Charlie," a source tells UsMagazine.com. "He has not heard from Charlie since he told him not to start New Year's day!"