What? to clear this up, you want the Vbulletin's CMS to show up when you goto
http://example.net ?
If so, You can create a redirect to the "forum" folder via a php file. Create a content.php or index.php file in the root of your web space "public_html" or "www" folder with the contents shown below.
PHP Code:
<?php
chdir('/home/USERNAME/public_html/forum');
require_once('/home/USERNAME/public_html/forum/content.php');
require_once('/home/USERNAME/public_html/forum/vb/bootstrap.php');
*** Where it said's USERNAME is where you need to put your hosting cpanel username in (if your using a cpanel type hosting server). Alternative is go into your cpanel and and look on the left side for "Home Directory". Whatever that said's you need to put into where it said's in the php file '/home/USERNAME/'.
If you need more assistance, Check the installation manual for vbulletin as it explains this better.
Hope this helps.