Think of it like an electrical circuit. I assume the way it works is that the VCV is like a variable resistor (rheostat) in series with a fixed resistor (orifice). That forms a "voltage-divider" which gives a "voltage" (vacuum pressure) signal to the transmission's vacuum modulator. Without the orifice, the transmission would get the full upstream vacuum (voltage source), less pressure drops from flow in the tubing. In 1985 cars, the "Blue Saucer" is a vacuum "buffer amplifier" between the VCV signal and transmission. Probably a good idea since they continued that 1986+, and why I put one in my 1984 300D (mounting holes were already there). The adjustable orifice by Alec300SD sounds good, if you spend the time to tweak it.

For those who don't know, the whole purpose of the VCV circuit is to simulate the normal intake vacuum signal of a gasoline engine. It works based on fuel pedal position. A bit clumsy, but the easiest way M-B could adapt these mechanical transmissions to a diesel engine. If Chrysler were doing it, they wouldn't need this since their Torqueflite transmissions get no vacuum signal from the gas engine. Ditto for GM I think. Seems old Ford transmissions had a similar vacuum signal since you see what appears to be a large vacuum piston on the side of their case, similar to the B-2 piston in the M-B transmissions.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans