RESPONSE
It is possible that the control head is faulty. I do not think a vacuum leak would cause an issue. The reason would be that Defrost is the default location for the airflow door. There is no vacuum applied when the mode switch is placed in the Defrost position.
How the Defrost, Floor, and Panel airflow operates on a manual system vs. Automatic Climate Control
The mode switch is used to direct output airflow. There are 5 different modes of operation: panel, bi-level, floor, floor/defrost and defrost. Different modes of operation are identified by symbols on the control panel. The switch controls vacuum signals to operate the air outlet doors.