Since you will be able to see the source code you can't really put anything meaningful in place. You just have to do what vB does and keep a record of domain names using your product and then if you find one that doesn't contact the site webmaster to ask to remove it or something.
You could encode the source with something like Zend Encoder or whatever it is called but I don't know how that will affect the running of the code and its integration with vBulletin.