Hi, I am not sure how to do this in vb, or if it is even possible, but how about the following:
If you really dont want people to see the location in the address bar, how about putting it in a 100% frame? Then place a cookie on your sites main page that expires after ? time, if the use does not have the cookie, they will automatically be forwarded from your forum to your main page. Once they have the cookie they can then visit the forum.
If you use the cookie method above then really it would not matter if they know the actual location, because without the cookie they are redirected to your main page.
Maybe someone else would have a better idea, just some suggestions.
You have a few options, although it sounds like you want the one that XT22 suggested.
If you put it in a frame then nobody will see /forums in the address bar.
You could also create a subdomain such as forums.yoursite.com
If your concern is blocking people from going DIRECTLY to the forums, then you can use mod_rewrite in a .htaccess file (assuming you are on linux, which it doesn't sound like you are) to deny access based on referrer.