It should be set to Yes.
Try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit includes/config.php and add this line right under <?php
PHP Code:
define('DISABLE_HOOKS', true);
Also, disable vbseo via it's own control panel and remove the .htaccess for it.
Now see if threads in that forum are moderated.