[postoronnim]

This is very depends on fields on your tables and data structure.
I have transferred to vbulletin a one custom forum. Before this migrating I have installed a fresh vbulletin with empty data and made a database snapshot. Then I set permissions, add a one user with avatar, one forum, one thread and one post with attachment and saw for the difference for each step. Also very helpful to download ImpEx Import System and to consider the import process for a one of simple forums (you can steal from it a necessary sql inserts and convert functions). After that I wrote scripts for capturing fields from old forum, converting their if needed, and inserting in vbulletin tables. One hint I remember: it is easer to transfer avatars&attachments if change their storage type from Database into File System before.