I would like to send Headphones and bus A to the same audio playback device. In this case to feed audio from our external intercom into an Audio input, route that to bus A, and then out to the headphones. This way the operator can listen to vMix and what the staff is saying over the intercom.

I tried, but it seems it is not possible to use an audio playback device for more than one Aaudio output (Master, Headphones, A or B).