Jennifer Lopez & Casper Smart -- Not Ready for Primetime

Jennifer Lopez and Casper Smart insist they're not back together, and Sunday night at the VMAs ... they went through great lengths to show they aren't ... which totally means they are.

As TMZ first reported, despite being photographed together last week ... her people swear they're leading separate lives and just friends.

So J.Lo and Casper walked the VMA red carpet separately, sat separately at the VMAs (except for a quick reunion for candy) and each showed up at Mastro's following the show ... but left separately.

Where there's smoke ...