is there a way to use the other camera on it? even if i connect it to an external battery?

Uhhh, seems like that would be a downgrade, don't you think?  You would completely lose the stability of the 3 axis gimbal that comes with the P2V+

That said, I suppose it could be done if you can figure out the pinouts of both the Vision camera and the Phantom.  You might try searching through the forum first, somebody might have already done it.

I do recall seeing a Quickrelease camera swap tool on Droneexpert but that setup would cost almost as much as a new quad!