Julianne Hough: More Ass, More Fun

Julianne Hough got her last licks in a bikini down in Mexico ... gracias dios!

Julianne, her sisters and their mom practiced the time honored tradition of "Sun's out, buns out" on their final day of vacay in Cabo San Lucas.

The only thing sick this time around are their swimsuits -- way better than the puke fest that went down the other day.

When they got back to LAX, JH and co. had an explanation for that mess ... if you buy it.