1) make sure that the heater hoses are not leaking at the heater core, otherwise, the core is bad.

2) There's a plastic lever for the air door on the left side of the HVAC housing that breaks. You used to be able to buy just the lever, but I think it superceded to a whole door/lever assembly. If you find it broken, the part that broke off will be inside the door in the housing. A long drywall screw will thread inside the broken part so you can extract the broken piece. If the new part is only available as a complete door, the housing will have to be removed and the case halfs split.

Hope this helps!

P.S. If all you lost was vacuum, but the housing was still operating properly, the system would default to the full defrost position, full heat as a safety measure.