Thank you for your reply, I'm afraid I have some further questions.
1.) So I can definitely import other vbulletin boards with impex?
2.) Where can I check out the exact versions of vbulletin and wbb supported by ImpEx, so that I know to what version I have to upgrade the old forums in order to be able to merge them.
3.) All of my forums have plenty of users that are registered on more than one of the forums. What happens to these identical user accounts when my four boards are merged? Does ImpEx prefix them somehow, so that I can merge those accounts in the ACP later?