[kh99]

The "That username is already in use..." message probably means the username is already in the database (or it could mean it's a username that isn't allowed), so up to there it looks like things are working as expected. I think you're getting the "Fatal error: Registry object is not an object ..." because the code is depending on including init.php to define $vbulletin, but you've got require_once() so it only works once.

To be honest, I looked at it trying to see how to fix it but I can't understand what's going on. Do you understand why it seems to be trying to replace the existing $vbulletin with a new one?