If you use one or the other, the simplest solution would be to change the default audio playback device in Windows sound properties to which ever you want to use at the time. Only if you want both outputs to play the same audio at the same time, would you need to use VAC. It's also just stereo as you say. There is no other way of doing it, AFAIK.
You can have different audio sources playing. Let's say a game using HDMI and maybe music using SPDIF, as long as the software has options to select a different audio device independent of Windows default. Many media players allow this. Certain games too.