Hi,
I recommend going to
http://vBulletin.com
and downloading the vBulletin demo. Find the template that you altered on your own forum and replace it with the unaltered pristine template from the Demo version.
My statistics had stopped acknowledging new registrations after i made a modification in the Navbar template.
After replacing the Navbar template with a fresh copy from the vBulletin Demo, everything reverted to proper working order. Just remember you will need to reestablish any other modifications you had made to the navbar template.
Best wishes,
_Brandr.