Huh, you can go to AdminCP->vBulletin Options->Forum Listings Display Options->Show Private Forums.
This is what manual has to say:
Quote:
Show Private Forums
Select 'No' here will hide private forums from users who are not allowed to access them. Users who do have permission to access them will have to log in before they can see these forums too.
Either of the yes options will allow users to see all forums that they don't have access to. The ability to see the forum's post count is controlled by the two yes options. If a forum is passworded, it is treated as a private forum until the user enters the valid password.
Well, it seems to me, you want last option selected.