you cant stop people from requesting blah.com/index.php
you could do an external redirect using htaccess, but your going to need to modify vbulletin so that all your links dont include the index.php? part....i cant help with that
but heres rewrite rules that would work if all you care about is the default pages
Code:
----.htaccess-------------
RewriteEngine on
RewriteRule ^forums/index.php$ http://%{SERVER_NAME}/forums/ [R,L]
RewriteRule ^index.php$ http://%{SERVER_NAME}/ [R,L]