UFC's Paige VanZant -- Punchy In Vegas ... In Little Black Dress

Here's UFC star Paige VanZant playfighting with 2 other MMA chicks in a Vegas casino ... in tiny black dresses.

Paige -- along with Cynthia Calvillo and Kyra Batara -- hit the Vegas club scene Saturday night ... and decided to do a little slap boxing to pump themselves up before rollin' through Surrender Nightclub at Encore.

Fighters gonna fight.