Nor should Vanitas deny Sora outright. He might be pretty guarded overall, but it's not like his desire to complete his heart is a secret. If anything Vanitas has made it clear that it's what he'd want more than anything and that he's pretty much willing to do whatever he deems necessary to get it. If Sora does manage to find a way then why not benefit from someone else's hard work?

"Not all of your little friends will agree with your choice," Vanitas snickers. "What will you do if they disapprove? With a completed heart I could just make things worse."

Ideally that's not the case. In fact the Unversed may cease to be. No one really knows what is going to happen if Vanitas actually does heal the wounds in his own fractured heart.