[CarCdr]

This should not be as difficult as you might think. A virtual forum could easily contain only redirects to other threads in real forums. vBulletin already supports that. Also, you'd set up the forums that are going to act as virtual forums to disallow posting by non-admin's.

All you need now is a script to run periodically that scans threads to see if they meet the criteria for inclusion in a virtual forum.

A few possible hitches come to mind:

1. You would not want to run this often as scanning the entire thread table, albeit for real forums only with threads that are new/changed, is time-consuming on a large board. Remember, you have to scan the entire table as even older threads might only recently meet the criteria (i.e., it did not match yesterday, but it matches today).

2. You'd probably want the ability for admin/mod to tag a thread by hand for inclusion in a virtual forum.

3. Would you ever want to scan the virtual forums to see if threads should be booted because they no longer meet the criteria?