Q. Why is the user database not shared between boards?

A. Because vbulletin does not support it directly within their software, nor will they support it through technical support. They tell me that it would be a hack, and they do not support hacks, though they were happy to point me to a hack site.
I went to the hack site, they asked me to get confirmation that I owned 2 licenses (I do) and then it was dropped and I never heard about it again.

If you think this is a bad policy, here is the contact page for jellsoft, the company that made vbulletin. http://www.vbulletin.com/contact.php