It was no problem to create the appropriate shader in Shader Forge.
However, the official undo function for Unity Editor scripts slowed down the plugin considerably, so it was not usable.
The solution was to code its own undo function, caching the modified Vertex Color arrays in lists that can then be read when needed.
This leads to an interesting construct of lists in lists in lists that fortunately is working smoothly.
if (undoLevels.Count > 0)