Matt Stafford's Fiancee -- BACHELORETTE PARTY ... Suns Out, Buns Out

It's 48 degrees and cloudy in Detroit ... so Matt Stafford's future wife hit Vegas for her bachelorette party -- and proved her ass can hang with the best of 'em!!!

Kelly Hall and her gal pals hit the pool party at Wet Republic at the MGM Grand ... where she did some bikini crowdsurfing ... bikini cheerleading ... and other bikini-clad activities -- under the 85 degree Vegas sun.

The Detroit Lions quarterback and his college sweetheart got engaged last year ... and they famously broke the news at a Cold Stone Creamery.

Now, the wedding is right around the corner -- hopefully, the reception will be half as much fun a the bachelorette party!!