I don’t know much about the Vision Pro, but based on Esri’s response, it sounds like the native code for VisionOS might be identical to the iOS code. If that’s the case, we just need to make Cesium for Unity use it. However, the fact that it didn’t actually work for the user in the end implies there might be more to it than that.
In any case, if you want to give it a try, you’ll need to make the same change in a whole bunch of scripts. In the Cesium for Unity package, open every file in Runtime/generated/Reinterop/Reinterop.RoslynSourceGenerator and replace all occurrences of #if !UNITY_EDITOR && UNITY_IOS with #if !UNITY_EDITOR && (UNITY_IOS || UNITY_VISIONOS).
I honestly have no idea if that will work, but it’s worth a try! Let us know.