[tnguy3n]

That'll work only for forumhome page, guests are still able to read threads and browse forums. And you can write a plugin to redirect all guests to register page.

Create new plugin at global_start hook with following content:

if (!$vbulletin->userinfo['userid'] AND THIS_SCRIPT != 'register')
{
	header("Location: " . $vbulletin->options['bburl'] . "/register.php");
}

Voila!