If you look in the manual, you see that everything above 2400Mhz is an overclock.
So the reference system would have 2400Mhz memory, because overclocking is not sure to be stable so should not be in a recommended system.
If you would choose to use higher clocked memory, chances are quite high you will not have a problem as long as you don't try to get to the edge.
If I look at memory prices, they get a lot more expensive when able to clock over 3200Mhz. So I would order 3200Mhz memory and test with it.
If it is working stable as a rock, keep it running like that, otherwise just clock it to 2400Mhz and go with proven specs that should get you no surprises, A "vMix has stopped working and needs to close" message is something nobody is waiting for in the middle of a production, after spending thousands of hardware. ;)

I do not have a solid answer on that, but what you want is stability over some extra memory bandwidth.
The obsidian system has the power vMix needs on stock memory settings, so getting higher clocked memory is on your own risk.
It surely doesn't need it to perform well, only when you push it to the max like the 4K60 webcast Martin and Tim did, it might get you a little more performance over stock speeds.
If you are not doing that and have like 8 HD inputs, it won't bring you anything more.

If you want to experiment with it anyway, I have the experience that tuning the memory timings, especially tRas (row active time) can get your results more stable.

The row active time sometimes needs to be set a little higher than in the profile on the memory to get better results. If the active row is closed before the entire row is read, it has to do the procedure again and with that you can get spikes and drops. Lots of times the profile has a tRas of all other timings and delays together, but needs just a little more to not close the row before it is fully read.

You can google about it, or read the first result I got that explains timings: HERE

The more memory you get, the harder it gets to drive it with overclocked speeds by the way. That's why you find mostly small amounts of memory on the compatibility lists of the mainboard manufacturers at high speeds.
The best option when installing a lot of memory is to take registered non ecc memory. That has a register that does all the work selecting the rows etc of the memory chips, but that only comes at stock speeds as far as I know.