I can’t give you complete instructions, because I don’t have a Vision Pro and have never tried to build for it. However, one thing that is certain is that building for a new platform from the release package will not work. You need to clone the cesium-unity repo and build it from source. The basic process for doing so is described here, but it really only covers Windows:
Another useful resource is our GitHub Actions workflow, which shows how Cesium for Unity is built by our CI system for macOS and iOS:
A key part of that script is dotnet run --project Build~, which invokes this script:
And that script, in turn, runs Unity from the command-line several times in order to build the project, which ends up running functions from here inside Unity:
And, via Unity magic, parts of this are invoked during that process in order to build the native code: