It's a smart move not to turn his back on Vanitas. If it ever comes down to a fight, the guy isn't above playing dirty and cheating. He most certainly is the time to strike when someone doesn't expect it either. Whatever it takes to win, honestly. Vanitas will do what he thinks is necessary in order to achieve what he wants. To get what he wants.

"Why would I do that?" He asks instead. "Most of us weren't like you and your friends. We don't need each other."

The question itself is stupid and the way that Vanitas sounds downright offended should give Sora a clue he has no intentions of it. Being a member himself had been the only way to further his own ambitions. He doesn't care about any of them. In fact he hadn't even cared about what Xehanort had intended to do. Vanitas rolls his eyes, ready to settle this stupidity.

He puts his hands on his helmet, taking the entire thing off this time before actually looking at Sora.

"I wanted one thing and one thing only. To forge the χ-blade. Whatever was going to happen after that didn't matter to me," he frowns. "We did just that, didn't we? So, Sora, what makes you think that I'd want to team up with those losers a second time?"

Though in the end, Vanitas never got what he really wanted: to be complete.