Impex works great, not sure who's been telling you the success rate is low.
I actually
merged 3 vBulletin databases last week with no problems at all.
Move the files on the target board into the database before making your backup, or you can leave them in a file system and include a path to those files during the import.
I've imported several scripts when I was doing vB work, never had a problem that couldn't be solved.
Read over the documentation linked above and you should be fine. Ask any questions in the impex forum on vb.com or open a support ticket.