I think the online vbulletin script works with cookies and with a timeout option. The onlinescript by Slip counts all userid's in "session". So the results are different.
Compare in index.php from vbulletin:
Code:
if ($displayloggedin) {
$datecut=time()-$cookietimeout;
//$loggedins=$DB_site->query_first("SELECT COUNT(*) AS sessions FROM session WHERE lastactivity>$datecut");
//$totalonline=$loggedins['sessions'];
$loggedins=$DB_site->query_first("SELECT COUNT(*) AS sessions FROM session WHERE userid=0 AND lastactivity>$datecut");
$numberguest=$loggedins['sessions'];
//$numberregistered=$totalonline-$numberguest;