I'm pretty sure this is the first time I have tried installing any Voukoder product, even Non-Pro. However, here is some additional information that may be helpful and could be the source of the problem.
I thought that I already had FFMPEG installed on my computer because I frequently run it from the command line. However, when I installed Voukoder Pro and ran Vegas Pro for the first time, I got an error message saying "you must first install FFMPEG before you can run Voukoder." So it was only at that point that I installed FFMPEG v7.1.1.
When Voukoder didn't show up as a Vegas render option, I of course tried "repair", "uninstall" and re-install for Voukoder.
So, could my clumsy installation have gotten the Voukoder/Vegas plug-in startup process in a bad state?