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FudForum has a PHP "hook" for integration with other login systems. We would need to look at this in more detail to understand how easy it is to use: http://cvs.prohost.org/index.php/Login_integration
I am not sure what integration vBulletin provides. There appears to be discussion of an LDAP plugin but it is not clear what level of support this has?
http://www.vbulletin.org/forum/showthread.php?t=231909
Ability to block external search engines
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Importing from other systems
Perhaps there is an FudForum FUDForum-> vBulletin >vBulletin importer. On the other hand the links in this post all appear to be broken (commercial support at work?).
http://www.vbulletin.com/forum/showthread.php?151319-FUDForum-Import
There appears to be a script for vBulletin->FUDForum. Unclear if it works: