I seem to have come even shorter than you with displaying the vertex colors. My setup is a simple USDC with vertex colors which show up fine in Blender but renders pink in both Xcode preview and in Reality Composer. I've tried every consiveable constellation of the shader graph nodes VertexID, GeometryColor and GeometryProperty Would love to hear from anybody that has gotten vertex colors to work in RCP, even just for math operations instead of shading.

Found an answer somewhere online to get vertex colors to work through shader graph in RCP. I had to rename the attribute in blender to "displayColor". It was then shown in RCP