vMix has some shorcuts to turn on or off a layer input: LayerOff, LayerOn, LayerOnOff. But incomprehensibly there isn't a tally or flag in the XML API output and neither exist an activator for this purpose.
For this same reason I can't find a "feedback" action on the vMix module for Companion which would be very useful.
This feature would be very appreciated for us as we deploy a solution with vmix and a unique preset using several (background) inputs with a chroma keyed camera input as enabled/disabled layer and it would be very useful to confirm in Stream Deck if chroma layer is on or off.