Paid Subscriptions are default a feature of vBulletin, see the manual for more information:
Paid Subscriptions
What we do here on vBulleitn.org is not something that is default, as our licensing check ties to a custom customer & licensing registration system running on vBulletin.com.
But if needed a coder might be able to create something for that ties to your licensing system.
PS Responses have been slow as it was almost impossible to post for a day on the vBulletin servers because of an issue in the NOC.