Perhaps it's only me, but many times I by mistake start vMix even though it is already running. Of course every time I soon find out when I try to use a camera input that's already in use. My first thought was that vMix should not allow multiple instances and say "vMix is already running". But perhaps I am missing some points with having multiple instances of vMix.

Any thoughts?

I believe vMix is only intended for single-instance usage.

I find running multiple instances potentially problematic as the external outputs that present virtual DirectShow capture devices to other programs would have a conflict, as each new instance of vMix would not have exclusive control over that virtual capture device. (I believe you only get one "vMix Video Device" for External 1 and one "vMix Video Device #2" for External 2 if you have the 4K version)

Additionally, there is only one set of vMix global app settings (user.config file stored in AppData\StudioCoast_Pty_Ltd) which could cause manageability/confusion problems with identical settings at launch for things like Fullscreen/Fullscreen2 screen position, etc...

Third, the vMix webserver binds to TCP port 8088, and if that port is already bound that service will fail to start. Changing that port changes the global app settings file, which will not really do anything good because other instances will eventually read from the same settings, maybe during runtime but definitely during initial loading.

While vMix could theoretically function in limited capacities in multiple instances, based on what I've learned with the program I would just build a second switching system because it will avoid a lot of headaches.