This is not about the hack. Even if it is vbulletin that is producing this error, it is not about vb either. It happens due to a server restriction you have and because you are trying to post data which is above this restriction.
Please see here for more info and also fix:
http://www.vbulletin.com/forum/showthread.php?t=78563
(If you are on shared hosting and can't get PHP memory limit increased, try to shorten your page)