Is the thumbwheel connected to the vent flap? If not, you can try reinserting the connecting rod to the flap with a small allen key (i forget what size, maybe 5mm). It doesn't screw in all the way, maybe about a third of the way - only until you can fully close and open it without too much effort.

If the thumbwheel IS connected to the vent flap, then the vacuum pods controlling the center vent flow may be busted. Fixing this is much more involved, and involves pulling out the dashboard..