Vertex Map Shader

Left a Vertex map, right Vertex Colors, each with Viewport and rendered versions.

The Vertex Map shader works well with Subdivision Surfaces — ensure that each Vertex Map tag has a unique name.

With this shader you can do the following:

Render Vertex Map tags as grayscale images. Areas with maximum weighting will be rendered white and areas with minimum weighting will be rendered black.
Render Vertex Color tags. Note that an alpha channel in a Vertex Color tag will only be evaluated for rendering if the same shader is also used in the Alpha channel.

Make sure to assign unique names to the tags.