Many forum admins wants users to know a forum exists but to not have access in order to entice users to register. This mod shows you how to show a forum while still not allowing access to it.

If You Don't Have Any Private Forums You Want Hidden
Restrict access to the forum in your:

Admin CP -> Forums & Moderators -> Forum Permissions

Set the Can View Forum permission to No. Then show private forums:

Admin CP -> vBulletin Options -> Forum Listings Display Options -> Show Private Forums = Yes

Now the forum will be visible but the user will get a "No Permission" screen when they try to enter it. This method is not good if you have other private forums that you want to hide from users because those forums will now be visible.
If You Want To Keep Some Private Forums Hidden
This method allows you to keep some forums hidden from view.

Restrict access to the forum in your:

Admin CP -> Forums & Moderators -> Forum Permissions

Set the Can View Forum permission to Yes and everything else to No. Leave private forums hidden:

Admin CP -> vBulletin Options -> Forum Listings Display Options -> Show Private Forums = No

You may also want to show a "lock" icon on the forum for users who don't have access, in which case you want to enable this option:

Admin CP -> vBulletin Options -> Forum Listings Display Options -> Show Lock Icons to Users = Yes

Now users will be able to see the forum but when they enter it there won't be any threads in the thread listing and they won't be able to post. Under this method you can keep some private forums hidden (Can View Forum = No) and others not.