What happens in Vegas isn't staying in Vegas -- at least when it comes to Britney's zombified performance at Sunday night's VMAs. Everyone's been wondering why the once unstoppable pop tart failed so miserably at a comeback performance, and Page Six has the answer.

Brit was supposed to begin rehearsals at 1 p.m. on Saturday, the day before the VMAs, but she was waaaay late. "She didn't even get to Las Vegas until 4:30 p.m. It was ridiculous," a show insider says. "The production people at MTV were freaking out . . . Nobody can tell Britney what to do anymore. No one can control her. She is a mess." But before she scurried off to run through her act, she made a little pitstop. "She went to her hotel room and ordered a bunch of food and some frozen margaritas," the spy says. "She came down, like, an hour later with a frozen margarita in her hand."

As for the rest of the dancers, they were prepared to put on a tremendous performance. "The dance number [at the rehearsal] was spectacular -- without her," the source spills. "When the stand-in was rehearsing with the dancers, in the hours they were all waiting for her, it was amazing. Then Britney showed up and refused to do anything. The dancers were supposed to lift and twirl her in the air a few times, and that just wasn't going to happen. The more complicated dance moves had to be erased because she couldn't do them." Ouch! This from the girl who used to rock the dance floor like nobody's business! Britney's fellow performers knew the show would be a trainwreck -- according to an insider, "The dancers were texting pals, asking them to pray for them. They were worried."

And as for the sparkly bra-and-underwear set she wore as a costume, that wasn't part of MTV's plan: "MTV wanted her to wear a corset outfit. It would have looked great and covered a lot of things up, but she hated it and didn't think it was sexy enough." Britney had brought the outfit she ended up wearing with her. (Hey, at least she was wearing underwear! Later that night, while she partied 'til 3 a.m., the paparazzi later snapped pics of the disgraced star sans undies.)

Our hearts and prayers go out to Britney, who's obviously suffering more than people realize. Please, girl -- get it together!