Your entire testing procedure is flawed.

The first thing to do is to connect the Mityvac directly to the transmission modulator line. Pump it up and see if it holds vacuum. A slow bleed is fine..........say 15" in 15 seconds..........but, anything more requires attention at the modulator.

Then, disconnect supply vaccum from the T that is above the VCV and connect your mityvac to this port. You'll have the Mityvac on one side and the line to the modulator on the opposite side and the line down to the VCV on the bull of the T.

Pump up the Mityvac and get a reading. It should be somewhere between 10" and 15".

Now, gradually open the rack and watch the gauge. It should slowly fall as the rack is opened. You might need to pump up the Mityvac during the test if the modulator is leaking slightly. When the rack is fully open, you should not be able to pump up the Mityvac much at all..........very close to zero vacuum.

That's a properly functioning VCV and it can be tuned to move the idle setpoint up or down depending on shift quality.

I'll be away for a few days and will see how you're doing when I return.