yeah. doing it through vbulletin would be much easier. check for the query that inserts the info in the user table (look for something like $userid = $DB_site->insert_id(); ) and then switch databases and insert the stuff you need into your site database. don't forget to switch back to the vbulletin database after that.
I understand it would be easier doing it through vBulletin but to be honest I really don't want to. I would think I could just copy and paste the necessary information to my register file and link it to the vBulletin database but I'm curious as to which files I would have to include in order to do that. Any ideas?