The software is coded on 32bit, so to say it's a bit dated is an understatement. This is the reason that they recommend Intel and such little system ram.
The software simply can't make use of all the ram in modern computers. That said i've been testing Ryzen Processors out for a while and haven't noticed any issues yet. The 3900X is going to to be way overkill though unless you are running other applications or services. The 3700X or 3800X would server you more than fine for a high end system unless you are doing some wild and crazy things. The big part of the equation is the GPU which vMix uses heavily. The problem is they are still using DX10.1 from 12 years ago. The truth is you could uses a 7700k with 8gigs of ram and a 2060 with 6g vram and not even use half the system in most production environments. Most Ryzen systems would be way overkill although i would recommend going with the current Zen 2(3000 series) based chips for their Single core IPC. Ultimately it's going to come down to what other programs or services you are running at the same time as vMix to truly spec out the system correctly.

Hope that helps.

Chad