Try this:
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
Then set this as the default and go and try to register on your site. Does anything show then?
(Do images show under Human Verification Manager > Image Verification Library for both GD and Imagemagick?)