Some suggestions:
1. Use vBulletin CSS classes in your <td>, <tr> and other tags. Also, use class="bginput" in your form elements.
2. Think about removing fieldsets and replacing them with something else because they are not correctly supported by some browsers. How about collapsible tables instead?
3. $db->hide_errors() and $db->show_errors() in install code, please. otherwise, there will be more errors than there have to be.
4. Bitfield rebuild code in installation and uninstallation codes, please.
5. Phrase what's not phrased in cpnav.
6. Some sort of access permission on the admin script? Otherwise anyone who is admin can access it.
Other than that, nice hack.

Although I don't think I'll be installing it.