Why needs to adjust.
A VCA is set at minimal leak through of the control voltage. A leak can be heard if you using a EGR with a fast attack as and no input audio signal at the other input. There is alway a leakage but in a normal situation with a signal at the audio input this must not be audible.
How difficult is adjusting the VCA
This is quit easy, you need only your ears. The only problem is to locate the VCA because the VCA doesn't have control knobs
Adjusting the VCA
Put a square fast LFO signal at the CV input. Listen to the output and adjust the trimpot for the minimum of leak.
7e PCB board with VCA1 and VCA2