Excuse me if this is a dumb answer, but why not load up your virtual instrument by right clicking and using “Add Instrument Track”?
Then you only have one virtual instrument track controlled by the slider. You’ll only have MIDI info there, but you can freeze it to audio whenever you want.
But I don’t know your workflow.
If it really doesn’t work I would sell it and look into the iOS app Cubase IC Pro, which might help you.