i have seen this thread here but wanted to state that I have a VT15 model and am experiencing similar symptoms. in short, switching between amps causes the volume to jump quite high. in my limited experience with it (i purchased it yesterday), the volume returns to the correct normal only after adjusting the volume knob.
i am able to change the effects knob with no change volume whatsoever.
it appears the thread i mentioned above is frequently updated, so that's a good sign. i just wanted to let it be known that my new VT15 exhibits very similar symptoms.