The C class gets it AC needed vacuum from the (central locking) vacuum pump in the right rear of the trunk.
It might be possible that somebody connected the vacuum line for the AC controls to the engine vacuum and that vacuum is not sufficient to control the vacuum valves involved.
There is a black plastic connector under the black cover behind the firewall at the right side of the engine bay and it might be possible that your AC vacuum is connected to that connector.
Check the vacuum pump in the trunk as well...you should be able to hear it pumping once in a while when you operate the AC system.
You are right about the default setting of the AC. AS soon as the car (and AC/heater) is turned off, the vacuum valves go back to their default position which is defrost (open vents along the windscreen and sides of the dash/side windows)
Does your C class have a vacuum adjustable ortopedic seat?
with a minor 600+k