I noticed that when you're in a sub-forum you can double-click on the left-most icon to mark all items in a forum as being read. However, this doesn't work from the main forum view.
In other words. It doesn't work here: http://www.defensivecarry.com/vbulletin/
But it does work here:
Is this a configuration issue? It took me a while to figure out why sometimes I had the option and other times I did not.