It's been a couple of years since I got rid of my 123, but I recall chasing down a similar problem. If the center vent actuator checks out OK - it holds vacuum and the vent opens - then you need to look for large vacuum leaks elsewhere in the system. There is one vacuum feed to the entire climate control system; it is shared by all five vacuum actuators in the system. The single vacuum feed contains a small orifice to limit flow; this is so that leaks in the climate control cannot negatively impact the power brakes.
Anyways, you need to look for a large leak in another actuator. The most likely culprit is the defrost actuator. The defrost vents are normally open; applying vacuum to that actuator turns off the defrost function. Hence to get cold air out the center vents, both the defrost actuator and the center vent actuator have vacuum applied.
As well, the defrost actuator on the 123 is a funky two stage device. It has two separate sources of vacuum, one from the vacuum block behind the pushbuttons, the other (I think...it's been a while) comes from the center vent actuator. Even though I had the 123 climate control manual, I could never figure out how/why it was built this way. I think the climate engineers at Mercedes are more insane than the rest of the engineers.