Make sure you are using all default vbulletin files (check in Maintenance > Diagnostics > Suspect File Version) and then use a default style also (you must set it to default in vboptions > style > default style):
Create a new style with no parent:
- Styles & Templates > Style Manager > Add New Style
- Parent Style: No Parent Style
- Title: Default vBulletin
- Allow User Selection: Yes
- Save
Using this style, with mods off and all default files, try to register - do you still have the same problem?