[nhawk]

While it may work, again you've over complicated what you want to do by adding 51 unneeded additional database queries each time forumdisplay is accessed.

Using the forumdisplay_complete hook...

if ($foruminfo['parentid'] >= 18 AND $foruminfo['parentid'] <= 68)
{
	$vbphrase['post_new_thread'] = 'Create a Listing';
}

The only thing that might complicate things is if your sub-forums have sub-forums. Then that's a different story.