A correct import should have been leaving the vBulletin database alone and having XenForo installed in it's own database. You should never have installed them into the same database or merged them in any way, except for the importer which would have "converted" your vBulletin data to your XenForo data.
Assuming you have no add-ons installed in your XenForo install... It could be technically possible to weed through the vBulletin tables and remove them. I've done that for someone who did the same thing I think you did... But it would be a little timely.