I'd head over to
http://www.vbulletin.com/forum/forumdisplay.php?f=59
and ask the question there. That's the vB3 troubleshooting forum
I've upgraded vB 2.3.0 boards to RC3 several times. The first time I had an error at step 16, but after repairing the database and running most all the maintenance options in the admin cp the next attempt worked without an error.
Also, vBulletin released a tool to see if your database was compatible with vB3 - it might be worth running
YMMV, but I'd repair the database, and run all the appropriate maintenance options through vB's admin cp. A clean database is liable to convert a little easier