I've been thinking of developing a hack, but my proficiency with vB is not all that great yet. So, I'll suggest this hack to you.
Right now I have about 15 forums divided into 3 catagories on my page. Currently unregistered users cannot view anything, when they try to load a page, they are denied with a custom 'no permissions' message. When I allow them access to view some of the forums, the forums that they can't view dont even show up, and thus they are unaware of them, and have no compelling reason to register to be able to view them. So, I allow them access to view the forums, but not the posts. Then, it just looks to them as if all the posts have been removed - again, no motivation. The next step would be to allow them to view the posts, and thus negating the reason why I require registration. (It's mature content, you see.)
So, if there were a happy medium in permissions, where users from a certain usergroup could LIST the thread topics, but not display the threads themselves, that would make things much more productive for the forum in question, while not completely denying access to all the other interesting, yet decidedly less private forums.