If you have air flow through the center duct then you do not have a vacuum problem. That is you don't have a normal vacuum problem. I have seen systems rigged to just do A/C where the vacuum ran straight of the motor to the heater valve and the center vent.
It takes vacuum to close the heater valve and to open the center vents so it likely is present.
The push button controller activates switchover valves (vacuum gates) which are in one group behind the glove box. There are six or seven switch-over valves and they open and close the vacuum to the various vacuum elements.
To test one should have a vacuum and wiring schematic.
Bosch Master, ASE Master, L1
33 years MB technician