as dfbonnet said, check out all the vacuum motors you can find for disconnected, cracked or collapsed rubber hoses. I may be wrong, but I think the vacuum motors are a round ball looking item with a small size rubber hose going into it. your programmer or the slide lever for the AC, if it has one sends the signal for the motor (round ball) to open or close a given vent selection. You may have noticed a sucking noise when you move from 1 setting to another, that is the vacuum motor being activated and that sound may be missing when you move to the regular vent setting.
Hope this helps If you haven't repaired it already...maybe if its fixed this will help someone else.
marcia