I just went through this on my 07 F250. I was having the defrost only-no vent problem. I learned on the forum to trace each vacuum line from hub to pump and make sure there are no disconnects or breaks/cracks in the lines. Up top at the pump, find the grey hose leaving the pump and trace it to the heater hose and make sure it is connected. I got excited and ordered a new pump before troubleshooting, and my grey hose was disconnected.
If all your lines are good, start the truck and disconnect the grey hose at the heater hose, put your finger over the grey hose and see if you feel a slight suction. You should. If you don't, make sure your vacuum pump is getting voltage. If all the above fails and the pump is getting voltage, order a new pump off Ebay. They are plentiful at $50-$75.
Hope this helps.