Takea look at these -
How to create your own vBulletin-powered page! (uses vB templates) or
[How-To] vBulletin API Basics: Creating Custom Pages & Misc. That will give you a basic php page that you can then add whatever you want into it. You may create a basic template and not include the $header or $navbar etc and just put your own html in there. You can steal the login code from the navbar template and put it in there.