Sofia and a bunch of her friends partied hard at the XS nightclub on the Strip, and it looked like a blast.

Scott Disick was there, but took a back seat to the action as Sofia danced wildly with friends. She went onstage with The Chainsmokers and just kept dancing.

There was the obligatory cake -- in this case, 6 tiers.

Sofia got some pretty awesome presents earlier in the day. Scott presented her with an Aston Martin and a Range Rover. Yep, 2 whips. When the cars were safely garaged, they jetted off to Vegas. Kylie was waiting at the airport where they all boarded the private jet.

In case you lost track, Scott and Sofia have been dating for about 2 years. They seem super serious ... she even hangs out with his kids, and she and Kourtney clearly get along. They've even all gone on vacay together.