Heidi's Alleged Sex Tape Partner -- Naked Sunbather

Playboy Playmate Karissa Shannon -- the star of an alleged girl-on-girl sex tape with Heidi Montag -- showed off the goods in Vegas this weekend ... during a quasi-naked sunbathing session.



As we previously reported, Karissa doesn't deny that a tape exists ... but she's not convinced that Spencer Pratt is actually in possession of it.