I'm currently making use of the full RGBA components of vertex paint, where RGB stands for color and A for blend height inside a shader.
The shader basically uses alpha 0.0 to 1.0 to set different textures, where 0.0 to 0.25 is one texture, 0.25 to 0.5 is another, and so on.
RGB is used for light baking, so far, it's pretty optimal and runs smoothly inside the game engine.
The workflow is quite difficult to follow mostly because blender has Add Alpha & Erase Alpha as default, not a float grayscale functionality, I'd be willing to pay for an add-on that lets me work with an isolated alpha channel in exact values, there is currently one, called Vertex Color Master, but it's a bit outdated, not working properly with alpha, and the developer doesn't have much time to work on it.
If you have any suggestions or ideas to set exact values in the Vertex Alpha channel, I'd be very happy to hear ideas or suggestions.
I'd love to request the alpha float painting as a feature for the vertex paint tool in blender instead of the add/erase combo, but I don't really know who or where would be appropriate.