There are two problems here:

First, is the sign up for a free forum. I would take that to mean that whoever signs up would get admin priviliges and all that stuff? Basically their own little forum world.

What you need to do is have a form, that when its submitted, the PHP3 code creates the entries in the VB control tables to accomplish this. (Not having seen them, I don't know exactly what they are...) This give a person control of a forum. The code would also likely need to create a directory, copy some files, and so on.

Unless I'm missing my guess, it should be relatively straight-foreward. I think the biggest pain here is the analysis to find out WHAT needs to be set and where.

Problem two, is detecting inactivity. This would be a process that could either be run as a task by the system (say 2 AM?) or by an admin that would basically scan your forums list for posts and build a list of things to delete.

Once it had that list, it would then procede to undo what process #1 essentially did. Or if you had put each forum in its own database, the delete becomes idiot simple. Just zap the database and remove the directory structure.

Both of these likely could be helped along tremendously by examining the VBulletin install script and could possibly even be modeled from it.

Hope this helps. If I had a live VBulletin running, I might be able to take a look at this for you guys.