I had an idea. I had thought since the Bonjour stuff was working, and I was able to make outgoing network connections, that my virtual network setup was fine. However, to support multiple machines, I was forced to not use the pre-built in mac address. So I had written a few lines of code to generate one randomly. But I thought, maybe there's rules about the bits in a mac address. So lo and behold, it turns out "use a random locally administered" address isn't just a good idea, it turns out it's also a function VZMACAddress.randomLocallyAdministered(), and when I created new virtual machines with a mac address generated with that function, then screen sharing works with nothing more than simply flipping the "screen sharing" switch in the virtual machine settings app. Including when I have a custom machine sharing name set.