In my experience and opinion, the VT doesn't really work as a stand alone drive to be kicked on and off, especially when you have distinct tones with your amp (in your case the GK). It alters the flavor of your amp too much and the eq'ing is so different that you'll always be chasing a "unity". It's great as a set and leave pedal in my experience - but I decided early on that it was unusable as a boost or drive pedal. If you're looking to just push your signal from a mild to a medium overdrive, I'd look into some boost pedals or I'd personally look at other drive pedals.
Leave the VT to what it does best - Ampeg flavor when you want Ampeg flavor.