[nhawk]

If all of your forums are already hidden, you can re-direct users that aren't logged in from the forum home page to the login page with a plugin...

Go to ACP->Plugins & Products.

Click Add Plugin

Product: vBulletin
Hook Location: forumhome_start
Title: Force Login Page
Execution Order: 5
Plugin PHP Code:

global $vbulletin;
if(!$vbulletin->userinfo['userid'])
{
	print_no_permission();
}

Plugin is Active: Yes

Click Save