i've fixed in this way
/includes/funcions_login.php
row #140
Code:
if ($vbulletin->userinfo = $vbulletin->db->query_first("SELECT userid, usergroupid, membergroupids, infractiongroupids, username, password, salt FROM " . TABLE_PREFIX . "user WHERE username = '" . $vbulletin->db->escape_string(htmlspecialchars_uni($username)) . "' AND usergroupid <> 3"))
any better idea?