[Xenon]

Well, there may be a way to achieve what you want:

Take a look at the modification forums here at vbulletin.org there is one forums which allows threadprefixes, but if you are within a thread the prefix can look as a subforums of the modifiaction forum.

i think with some changes to mystics hack, you can get exactly what you want

i don't think that you need 13k forums, as you won't set up permissions individually, so i suggest, to use the prefix hack, and get a coder to change it in a way to look EXACTLY like subforums