All you would have to do is forward the domains to those subforums. I'll review the Vbulletin T.O.S. In a second, but I believe you need one vbulletin lisence per installation.
Edit!- I was close enough-
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.
To pull something out of there so you don't have to read it all-
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.
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