It is due to some new security precautions added in vBulletin to prevent POST requests from other domain names from being sent to your forums.
Ive only seen this fix elsewhere and havent tried it out myself yet.
Let me know if it works so I can add it to the instructions.
Open your /forum/login.php file and look for this code:
Code:
error_reporting(E_ALL & ~E_NOTICE);
Below this add;
Code:
define('SKIP_REFERRER_CHECK', true);
Save and upload the file.