AC Ductwork Vacuum Leak???

My 2008 Econoline has an issue whenever I accelerate. Air conditioning switches from the dash vents to the defrost vents. When I decelerate, it goes back to the dash vents (normal). Fan speed doesn't matter. Any ideas? Are the dampers operated by a vacuum line?

yes- vacuum leak underhood most likely- trace the line- look for cracked or broken reservoir line..

Like he says,vacuum leak,most likely before controls. System is designed to default to defrost with loss of vacuum. Theres a vacuum tree at the top,center of the dashboard. Remove panel and check hoses. Check hoses going back to the engine compartment. If this is a diesel engine check the vacuum pump and it's hoses.