Kevin Federline, Wilmer Valderrama Rap in Vegas

Nothing says hard core hip-hop like Kevin Federline and Wilmer Valderrama.

TMZ spotted the pair of wannabe rappers taking to the mic at TAO in Las Vegas over Memorial Day weekend, dropping their own ill rhymes to whatever the DJ played.

In other words, they just keep saying "yo" as some guy spins generic rap beats. Too bad they couldn't get mini K-Fed to make a cameo.

The Vegas jaunt marked Kevin's first since April, where he was rumored to be getting wild at a poolside party - a charge an eyewitness wrote us to refute.

Rumor has it that K-Fed invited Wilmer Valderrama to appear on his next album. This assumes someone will ever produce another Kevin Federline album.

But still, how lame is that? And we thought "Lose Control" was bad.

We've screen-capped TMZ's exclusive video of it for you above, just so you can see we're not making this crap up. The exes of Britney Spears, and, well, just about every young celebrity in Hollywood (Mandy Moore, Lindsay Lohan, and Jennifer Love Hewitt to name a few) really did rap together on stage. And it rocked.

The unquestioned highlight: K-Fed screaming "I wanna hear all the bitches up on in this scream" ... and maybe one did.