[Scanu]

I'm not sure i'm just trying, you could search on showthread.php for the condition that give no permissions (line 238)

PHP Code:

// *********************************************************************************
        // check forum permissions
        $forumperms = fetch_permissions($threadinfo['forumid']);
        if (!($forumperms & $vbulletin->bf_ugp_forumpermissions['canview']) OR !($forumperms & $vbulletin->bf_ugp_forumpermissions['canviewthreads']))
        {
            print_no_permission();
        }
        if (!($forumperms & $vbulletin->bf_ugp_forumpermissions['canviewothers']) AND ($threadinfo['postuserid'] != $vbulletin->userinfo['userid'] OR $vbulletin->userinfo['userid'] == 0))
        {
            print_no_permission();
        } 


You have to remove one of these 2 conditions depending on how you set permissions for guests and users in that forum
If you remove this condition, guests and users aren't allowed to use a forum so they won't see the forum (and it's listed threads) on forumhome.php and forumdisplay.php but if they know the link of the thread then they won't get a no permissions message but they'll be able to see the thread