That is a very distinct possibility, isn't it? Vanitas could likely try something on both Ventus and Sora.

"Let's just say, if something happens to Ventus I would know about it almost immediately," he offers. "Depending upon what that is, I can't just let the person that has the other half of me just fade, right?"

Meaning he isn't quite ready to let go of his other half yet. Maybe Ventus doesn't need Vanitas to be whole, but the reverse isn't true for him. Strange as it might be, Vanitas is invested in what Ventus does and how he grows. Though aside from that, Vanitas closes the distance between himself and Sora. It's not really an aggressive move. He merely wants to jab a finger at Sora's heart.

"Part of you always will exist within Ventus and thus effects me. If something were to happen to you then that will also effect me."