See http://xenforo.com/community/threads/import-of-vb3-8-x-paid-subscriptions.63774 for a bit more color.
Basically, need something that will receive data from paypal IPN posts and update xF so a users subscription stays in place and doesnt expire at the end of a month/paid cycle. It needs to mimic vBulletin 3.X whereas paypal posts data in monthly/etc to say the recurring payment has been successful and to continue honoring the subscription. The paypal data url(s) tied to the subscription cannot be modified so if we want to prevent the user from having to re-sub, we have to mimic what vB was doing.
Lots of info on this across the community, but whoever takes on this development project will be a hero to the community and enable lots of big boards with paid userbases an easy migration strategy to xF.
Ping me privately or via this thread to talk further.