Weird! I did different install of this script and it is working okay for me.
www.tutorialistic.net/forums/testube.php
Hav you chmoded your youtube.php file to "777"? Also you have to upload in your forums directory ,i mean vbulletin forum directory.
EDIT: Are you using
AJAX Post Thankyou Hack? Try disabling all the plugins and try it. It is really weird. I have used this on 5 forums and all are working well.