Problem is in Slate, you still lack alot of functions to make the magic happen in a friendly way. You're stilling going to put a Blend into a Blend, and still need to manually access each node and set up the masking manually. It also doesn't help that you need to setup your Vertex and Texture slots (Bitmap) manually via…
ShaderFX for max will probably do it, but it is only in the viewport. I think Josh Stubbles also put together some kind of viewport shader, I remember he used it to blend grass and dirt... Now that max has Slate you might be able to wire whatever output node you want into whatever input node, so you might want to start…
You can use RGB Tint Maps to convert RGB Vertex Paint into 3 separate Masks...paint one Mask red, one green, one blue, and set the respective channels black or white in the RGB Tint Map (eg: red mask > red channel white, blue and green channel black)...use separate Vertex Paint Modifiers for each Channel and set the Blend…