I would say try importing the backup SQL file from the pbboard to a blank and new database on same server as the vBulletin board is on. Then point Impex to the new database as a target. You can then try different settings in Impex to see if it is able to grab the SQL that way. It can't do any damage - you are only working with a copy. If you are bale to grab it, great. If not, no harm no foul.
I would try this first though on my test version of vBuletin, if you have one, so you can see what gets imported and all, if Impex is indeed able to grab the SQL. If you like the results you can duplicate the procedure for the live site.