It would be handy to be able to use the full inputs UI, tabs and search to control the additional vMix mixes apart from the primary M/E.

This could take several forms; a button on Mix inputs that turns the default pv/pgm into pv/pgm for that mix (possibly with a visual indication around the pv/pgm windows) or a dropdown/buttons between the pv/pgm windows where one could select the mix you are viewing in pv/pgm and controlling with the transition buttons and previewing content as you would on your primary mix.