I built my adapter based on this image.
To my eye, this seemed as if the narrow side of the connector was at the bottom of the image.
However, comparing to this image, the error of my ways was pretty evident.
Anyway, long story short, I corrected my adapter and gave it another try. I needed to add a firewall rule specifically for network_configuration.exe, and execute that directly as Administrator to get the adapter configured in Vehicle Mode.
I've got the green indicator of success.