I think this should probably be in General Discussion rather than a Feature Request, but to answer your question:-

Add two audio inputs, one for the Mic in, and the other for the "Stereo Mix" recording device (which is the "system audio"), but be careful not to create a feedback loop with the vMix audio output also becoming part of the system audio. To do this, go to the vMix Settings for Audio Outputs and make sure none of the outputs are going to the "Default Windows Playback Device" (or to the named device which is set as the default, e.g. Speakers <Realtek High Definition Audio>).

See also:- Recording System Audio in the vMix Knowledge Base

One you have the two sources as separate inputs you can create whatever mix you want.