As you may know, the circuit from ECU Pin 20 goes to the Cluster at T32/3.
So, I am not sure what you are trying to determine. If it is 5 volts or 12 volts, you cannot change it. Other than tapping into that circuit, I have no clue how you could determine the voltage.
Anyway, when I installed the ALH TDI engine in my Vanagon, I tried to incorporate as many of the bells and whistles as possible. For example, I modified the Jetta Cluster to fit in place of the Vanagon Cluster.
In reference to the VSS in my Vanagon application: I screwed the VSS head onto the Ticker Box of the Vanagon (perfect fit) to obtain wheel rotation input. The VSS gets 12 volts from Fuse #7, per the schematics. The VSS output signal (no clue the voltage) goes to the Input on a Dakota Digital Interface that I used. I used Switched 12 volts to the Dakota Digital Interface. Most likely with advice from someone who knew, I chose the 2000 ppm Output signal to the Cluster at T32/28. The Dakota Digital Interface has Up and Down dip switches to calibrate the signal going to the Cluster. With the help of my wife, we drove down the highway using GPS to let me know how fast the Van was moving. We set the speed spot-on. Luckily, the odometer is about 99.99% accurate.
So, I surmise once the Cluster receives the signal from the VSS (in my case, the Dakota Digital) it uses it to operate the Speedometer. And, at the same instant the signal goes out 32/3 of the Cluster to the ECU at Pin T120/20. The ECU uses the data to operate the Cruise Control.
I know this doesn't directly answer you question. Maybe someone that's monitored the voltage or have a known source of info can provide the answer you seek.