I haven't used my copy of Vegas 12 for about a year and a half (July 2022), judging by the log files. It got to be a hassle changing the connector back and forth (uninstall/reinstall) between 12 and 15 every time.
But I know it did work with Voukoder at one point, and I'm pretty sure it was working last time I used it a year and a half ago. Tried the Voukoder/connector versions from that time period, though, and no joy.
Maybe something changed with Windows folder permissions etc? It's odd that no connector versions seem to work with Vegas Pro 12.
In "About Vegas Pro" -> "Components", voukoderplug.dll is listed. So the connector appears to be properly installed.
I did do a fresh install on a different computer, and behavior is identical.