Wordpress showing 404
 

I have a blog as the main domain of my website. mydomain.com.

I want to use vbulletin as the forums for that site:
mydomain.com/vbulletin

However WP is forcing a 404 because it thinks /forums is a dead WP page.

How do I allow WP and vbulletin to work together? I think I need to edit something in the .htaccess but I am not sure what.