Or, he was just doing it slowly for dramatic tension. That, or it was to tease Ven into fightiing with him, and thereby forging the X-Blade.
I don't see how any of that proves he magically had an awesome hidden agenda.
He was weakened, so he fled. Or, you know, he decided to save it for later, since it could help motivate Ven to fight him, and help push Terra over the edge (pretty much exactly what Braig said he'd do, more or less).
Vanitas has the body of a fifteen year old. He is not nearly as strong as MX or his heart. Xehanort's Heartless is far stronger than Vanitas is.
You still have no proof of that. Him destroying Vanitas completely would also make more sense for the story. Destroying Vanitas would mean that Ven destroyed a part of his heart, which makes sense as to why Ven's heart becomes fractured again.