Well, I think it would have to rely on cookies to identify that the same user is visiting each site. Since cookies are only sent to the domain they originate from, you can't just set a cookie to keep someone logged in on all domains. My understanding is that this problem is normally handled by having one domain handle the cookies, and when a user visits one of the other domains and they are not logged in, they would be redirected to the cookie-handling domain which would then redirect them back with some informaiton to log them in. It could be done in vbulletin of course but it would take some programming.
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