I double checked, and it looks like vMix hasn't added any kind of marker feature to the software, and I know a lot of folks would like this. I know I did. And now, I have something!
First, the theory:
Back in the day, before digital timecode lock, there were systems that would add an audio track with a timecode clock in it. By sending this to all cameras, you could sync them up perfectly using this audio in post.
That got me thinking: could I put an audio pulse into additional audio tracks in my recording?
It took a few years to get to it, but I've finally done it, and here's how it works:
1) Find some kind of audio file that is a pulse. Could be a clap, snap, tone. I used a mouse click. All that matters is that it makes a spike in your audio, just like a sync clap would.
2) In vMix, import the clip and set it to output on alternate audio channels. I set mine to buss B, I think. It doesn't matter, but what's important is that the clip audio doesn't go into your master audio buss, otherwise the click will show up in your main audio.
3) Now, in the audio mixer, set the buss you chose to any channel(s) you're not using for your main mix. My main channels are 1 and 2, so I used 3 & 4.
4) Set your video to record more than two audio channels. For me, I am using an external recorder (so I don't have to wait for file transfers when I take the video home for post; I use the internal recording as a backup). I have an Atomos Inferno, and set it to record four audio channels. I assume there is a way to do this internally in vMix for local recordings, as well. Otherwise, you may have to run two recordings at the same time, routing the appropriate content to each file, then syncing later.
5) I then have a shortcut tied to that clip, so I can hit a key (I use a control surface, but this can work with a regular keyboard), and then the clip will play in the alt audio channels, which creates a reference spike!
Now, every time something happens that I want to mark, I just hit that key. Obviously, it can't mark exactly where the event happens, it will always be a little after, but that's enough to help me find it! And, if you wanted different kinds of marks, you could load in different clips, possibly with different audio levels to make the difference easier to see, and then you could do all sorts of marking!
Anyway, I hope that's helpful or at least interesting!