The <form> element have a method attribute, where you can put "get" or "post". When you submit a form with "get" there the variables will appear in url but with "post" attributes will be sent to server in header as attachment.

In serverside php is making two arrays one is $_POST and the other is $_GET. There is also $_REQUEST which is combined from $_POST, $_GET and $_COOKIE. If vbulletin gets login data from $_REQUEST your url should work, but if it uses $_POST you just can't do that.

Try FF addon `live HTTP headers` to see what data are you really sending to the server.