[dartho]

Thanks for the feedback Floris, but I think you may be reading too much into the license agreement..

vBulletin license grants you the right to run one 
instance (a single installation) of the Software on one 
web server and one web site for each license purchased. 
Each license may  power one instance of the Software
 on one domain. For each installed instance of the
 Software, a separate license is required. Modifications
 to the Software or database to circumvent the 
one-license-one-board rule are prohibited.

Based on the wording of the license agreement, sub-domains are fine - no matter what you do with the style. There's no mention of "appearing as a seperate instance". The basics appear to be: One License, One Instance/Installation, One Domain. A domain includes sub-domains, thats one of the many things that makes a domain a domain.

With a vanilla vBulletin install, you can already assign different theme's to different forums within your board. vbSEO maps forum names to subdirectory names - I'm suprised subdomains have not yet become part of vbSEO also, subdomains are quite an effective way of optomising your 'crawlability'

I'm going to check with vBulletin and point them to this thread to clarify.