[magnus]

How is the .htaccess file written to currently? Do you edit the file manually, or do you have a PHP interface?

If you use an interface, an extra query could be written to add a user to BOTH the vBulletin database as well as the .htaccess file -- this would also bypass the need to 'register' on the forum.

* Edit: I re-read and see that it's written to by your credit card processor. Is this script something YOU have access to? (ie. Can it be modified?) If so, what you're asking for would be relatively simple.

One, less enticing -- but overall more congruent method is to use vBulletin as your backend for everything. With vBulletin's subscription system, you can manage subscriptions to both the content and forum -- thus affording you the ability to use vBulletin's powerful conditional and access management system as well as a central userbase for whatever else you may wish to add in the future.