I don’t understand what the problem is and why this question turned out to be in this topic.
If you have a static signal (for a call), you can assign it as an alternative output in the settings and assign it in the call window. If it is dynamic, in the alternative output you specify a preview and assign it to the caller. For 2 years of working with vMix calls, there have never been such problems.Of course - if you have a master exit set for the caller, then when communicating (in the process), he often sees himself and not the person he is talking to.
You need to understand the setup and more.

I often encounter a problem when connecting subscribers with MAC or iOS devices - when the security settings do not allow you to create a connection.