Since you failed to provide a vehicle year and model, I'm going to assume that the vehicle is the '83 W-123 in your profile.

With this VCV, you can do a quick check of the valve by connecting the Mityvac to the top port. Pump up the Mityvac and read the gauge. If the valve stops at some point between 8" and 15", you're off to a good start.

Now, watch the gauge and slowly open the rack. The gauge should fall off proportionally to the amount of rack opening until it finally reaches zero near the point of maximum fuel.

While you're checking with the Mityvac, connect it up to the modulator line to the transmission. Pump it up and see if the modulator holds vacuum. A slow leak (15 seconds) is acceptable. Any greater leak is not acceptable.