As recommended by Rob Watts and others on this thread I set the volume on Mojo 2 to dark blue in the second volume range, so getting close to max volume. I think this is equivalent to 3v but I'm not certain
This has worked well for other amps I've paired the Mojo with, but with the Vorzuge it was just too loud and I could barely get the volume knob above 8 o'clock.
So I'm now dialling it back a few notches to get a greater range of volume control on the amp itself.
Don't think there's a hard a fast rule with this, a little experimentation should yield the optimal setting for you.
Do you have a adjustable gain setting on the Vorzuge amp?