Frankly, I don't have an answer for this one. I noticed they weren't appearing so I checked the vBulletin settings and everything was turned on so I thought it must've been a FireFox issue. After checking out the site in IE as well I realized something else was going on.
To make a long story short... just going into the vBulletin control panel and, without making any changes, saving the settings again turned the DHMTL menus back on again.
Very strange.