@Scout Your one year old PC probably came (like mine) with an early Win 11 build and subsequent upgrades have kept the Windows AC3, as mine has. I have no problems with AC3 both encoding and decoding with VP21, which is similar to your experience with VP22 Build 122.
As to why the latest VP22 builds containing the Vegas AC3 encoder don't work for you is a mystery.
@EricLNZ I'm updated to build 250. The files open in Vegas, but even though the audio track is visible, there is no sound. The files in question are MPEG-TS files, so I tried to download and import them as MXF files instead, and that worked! Good video and audio, and I finally have build 250!