Using Impex only to upgrade vbulletin is a non-sense thing. You will corrupt all your internal links, you will lost search engine ranks and you will have a lot of problems with GARS.
I'll try to explain...
The main problem is that every time you deleted a post, a thread, a user or a forum you generated a hole in the index-id column in the mysql tables. Impex will insert your old data rows in a new table with an unpredictable offset.
So the best thing is to
follow
ragtek's advice.