Welll looks to me like on delete of a special usergroup that it doesnt clean up members memgroupid from the user table. At any rate that as stated is a
vb issue.
In vbulletin membergroups are stored in the user table
$mgroups = $results['usergroupid'];
if ( $results['membergroupids'] != "" ) $mgroups .= ",{$results['membergroupids']}";