You should not be using the (old) AJAX object. vBulletin now uses the YUI Javascript framework. You can fire a request with just:
Code:
YAHOO.util.Connect.asyncRequest('POST', scriptpath + '?do=ajax', {
success: this.handle_ajax_response,
failure: this.handle_ajax_error,
timeout: vB_Default_Timeout,
scope: this
}, SESSIONURL + 'securitytoken=' + SECURITYTOKEN + '&foo=' + foo);