imo I think a lot of the VMA stuff boils down to how Britney just didn't seem to be interested in the performance. (not criticizing her because at that point of her life, how could she be? there's a reason to why Blackout had zero promo)
At this rehearsal, she looks barely interested, it's little better than the actual performance. Not to mention the story of how she skipped rehearsal to go partying and all that.
I think that what happened backstage is that she got really nervous after she realized how underrehearsed she was. Sure, the Sarah Silverman stuff may have only made it worse, but I think that in the end it all boils down to Britney's lack of interest in promoting Blackout back then