Hi -
Without seeing your full material setup, I can only wager a guess as to the root cause, but even in vertex painter mode, the engine will attempt to show what the material is doing in the vertex shader on the applied mesh. Do you get the same errors on a large mesh using a material that is Opaque Unlit with just Vertex Color plugged into the Emissive Channel? If you do not get the error, then it probably is something that is happening in your material in the vertex shader components, like WPO or Custom UV settings.
Thank You and let me know -
Eric Ketchum