The whole subforum thing is proving to be little bit of a challenge, without causing problem in other areas of the forum, I'll get it worked out eventually, but its gonna take time and I'm short on that right now.
The vBulletin software come pre loaded for sub forums, but its for those who cant really cut it when it comes to creating and modifying a forum, kind of an easy way out.
I have set up some usergroups that can be accesses at the top of each page by clicking usergroups, find the one that fall into being your category and see how that works out, while I continue to work on the subforum software.
Thanks