Yesterday night I finally finished converting my tForum to vBulletin. I wrote a php script to transfer the posts / threads form the tForum database to the vBulletin database (for increased difficulty, I have two seperate databases ;) ).
You must have transferred the user database which should be a cakewalk using phpMyAdmin.

To use this script and transfer your databases, do the following:

1. Check the ForumID of the source and target forum.
2. Modify the transfer.php script by changing database, username, password, source and target board ids, and the userIDs in the 'IF' - section.
3. Point your browser to the script. If you didn't get any errors, everything should be fine.
4. Repeat step 1.-3. for every board you want to transfer.

I know the script is kinda akward to use and it could of course be much more comfortable, but it became late yesterday, I was tired and not motivated enough to make this script nicer to handle. Sorry ;)