That heart was indeed Sora's, because it caused Vanitas to resemble Sora. And because Ventus entered Sora's heart, that altered Roxas' appearance.

It would seem as though Ven had better control over his darkness than Terra did, which would appear to me that his inner light was dominating his heart, so the only was for Xehanort to bring it out was to extract it, because Ven was too reluctant to use that darkness. Once his darkness was extracted, Ven's heart was pure light, just like Kairi's heart.

So, in your opinion, even though the connection between Aqua and Sora is certainly tenuous, like you said, do you still view the connection between Ventus and Kairi to be a little less tenuous than the one between Aqua and Sora?