Juan Pablo -- I Got Me a New Bachelorette ... At Least for a Night

Juan Pablo was up to no good in Vegas -- at least that's the way it seemed --- cavorting with a soon-to-be bride until all hours of the morning.

The most hated bachelor ever on "The Bachelor" was prowling around The Bank Nightclub at the Bellagio last Saturday, when he spotted a bachelorette party ... not the show, just random chicks.

Juan was like a heat-seeking missile, zeroing in on the bride, taking her on the dance floor where he got all handsy and grindy.

But Juan wasn't done. He invited the entire party to his table, but they all declined ... except the bride. We're told the 2 of them drank and danced until 4 AM.