What exactly does the thermo vacuum switch do?(first switch by the top front of the engine/valve cover with two vacuum lines attached to it) and, how do you test it? I can not find the info on it and, I think mine is leaking...by the name itself, I suspect it applies/cuts vacuum to ? at a certain temperature.

You are correct in your assumption. On the side of the valve in very small print is the open/close temps. There is no in or out. It is rather open or closed. It should do the opposite at the open/close temp points.

Thanks Benzmac! but, what does it control? or, in other words, if it does not work. How is the engine performance affected?
TIA

The thermo vacuum switch on mercedes diesels let the car make more stationary rpm's during a short period of the engine being cold. After a short time the engine gets gains a little in temperature and the vacuum switch closes and the stationary rpm's get less.
I do not know whether the purpose of a vacuum switch on a 300E is the same.