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To date there have been no modifications made to include vBulletin with Xoops.
The only way around this is to install phpBB, then use a script to migrate everything from vB to phpBB then use Koudanshi's phpBB module for XOOPS which can be found.
If you attached to vB then this option may not be desirable because of the amount of work involved to be sure everything works. Then you you be forceing your loyal visitors to use a new system, which of course is somehting you would have to decide for yourself.