I have been using the MultiTrack Audio WAV record feature recently.
Unfortunately when you use this your Master Audio is not recorded onto the MP4 video.
Which makes sense since I told vMix to record 8 tracks of audio and now MP4 is like I only do 2 so it records no audio.

This leads me to needing to re-time or re-record with Audio and Video.
Easy enough to drop into some softwares timeline and then render.
BUT it can take a while depending on what software and system.

This led me to think can you link A & V or can you 'link' or 'group' anything in vMix?
I was able to achieve what I am trying to do by 'playing' both at the same time BUT they may get out of sync over time.

It would be interesting to see if you could "Group" Inputs.
That way with a single command it would effect(affect?) the group.