Domain Root And Subdomain Structure

I am a bit confused about domain and sub-domain setup for my website.

I have Wordpress and VBulletin Forum on my website like this:

/www/mywebsite/wordpress
/www/mywebsite/vbulletin

Right now, my domain points to this folder:
[url]

I have a .htaccess file there which redirects it to the wordpress folder i.e.
Redirect permanent /index.php http://www.mywebsite.com/wordpress
DirectoryIndex index.php

But, I would like my domain name point directly to the wordpress folder like this:
[url]

But the problem when i do that is my "VBulletin" forum get's messed up because then:
[url]----------resolves to--------->
[url]

I m thinking to resolve this issues, I will have to add a subdomain for forums like this:
[url]-> [url]

But I am wondering if I do that, won't it fail again, because it will do this:
[url]-----will resolve to----->
[url]