Hello. If i understand well what you need to do.You are the vmix host in place A. One caller is in place B and another in place C and you want these 2 persons to be able to talk between them. For this, you dont need any special setup. Simply start your 2 vmix calls with default settings and everything ok. You can monitor their audio from vmix as you usually do

How are you getting the host his audio? You should set up a separate audio bus for the host that has only the guest audio. Send that audio bus to a device and connect the host's headphones. Your controller headphones should be a separate audio device.

If you want to
(a) send a headphone feed to the host so that they can hear the guest, but not themselves,
and also
(b) send a mix that includes the host audio, either to headphones or speakers, so that the main program output can be monitored (by someone other than the host),
then you need a minimum of two audio output devices, one for each audio mix.

In the audio settings image that "talkinlive" has posted, there only appears to be one audio device, the "Default Windows Playback Device" - and it's assigned to both the Master Output mix and the Headphones mix. In my opinion, this is not a desirable configuration. To get proper functionality from having two mixes available, the Master Output and the Headphones, they should not be mixed together into one device output, or else it can get confusing as to what the controls are supposed to be doing.

If you don't have a second audio output device, then I suggest obtaining one. e.g. a USB audio interface, or if you've got one already and that's what you're using as the Windows Default, then use the built-in soundcard as the second device. Once you have two physical audio output devices taking both the (virtual) Master Output and Headphone mixes, it's a simple task to use the Master Output device to hear all the audio, and to use the Headphones output for the host (with the headphone volume control turned down on their audio input channel settings so they don't hear themselves, but do hear everything else.

You can of course use any of the audio busses (A, B, C, etc) for the feed to the host, instead of the Headphones bus, and have your second audio device assigned to the output of that bus instead of the Headphones bus. In that case, set the Headphones output to None.