Nice mod and a few suggestions.
The option to change the default e-mails so that the register link includes the person referring them, Example:
To sign up please visit the following page:
http://www.domain.com/forum/register.php?referrerid=$userid
This way vbulletin automatically adds the member who invited them as a referrer on the registration page.
Also please change the inactive user deletion to have addition confirmation warning e-mails send before the deletion is complete. So hopefully less members are lost.
Thanks!