OK, if you don't want to start with sendmessage.php, then start with one of these -
How to create your own vBulletin-powered page! (uses vB templates) or
[How-To] vBulletin API Basics: Creating Custom Pages & Misc. - then google "php forms" and you can write your own without the standard subject (although why you wouldn't want to just remove that part from sendmessage.php instead, I don't know).