*NSYNC -- Definitely Performing at VMAs ... Probably

It's happening ... it's really, really happening ... *NSYNC is reuniting on the VMA stage -- this according to the band's official Twitter.

It's kind of ironic ... Twitter didn't even exist when *NSYNC was famous -- but that didn't stop the boy band from all-but-confirming their HIGHLY anticipated MTV performance in 140 characters or less, writing, "mic check, mic check one ... two ... is this thing on?!"