This is a reply I got from Jake / vbulletin team on vbulletin.com
http://www.vbulletin.com/forum/showthread.php?t=178165
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Sharing the same userbase is much more difficult. For help with this I recommend you post on
www.vbulletin.org.
From your description of what you want to do, I recommend you look into using different styles on the same forum that are activated by a plugin or file hack by the different domains.
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