How to mask textures by vertex color?

I want to know if one can apply a decal texture to a vertex, just like a mesh can have vertex color on every vertex. This could be useful for simulating damage using another texture that will be visible on specific vertices, usually the contactPoints[] from the collision.

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My approach would be to use vertex colors as a mask to determine which vertices will get the secondary texture (white color = MainTex, black color = SecTex) and then map the textures on the mesh, giving their visibility based on the color of each vertex.

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A cool effectwould be that the textures would nicely blend due to the resulting grayscale blending of the vertex colors.

Can this work? And how can I write this kind of vertex-shader.

Any help appreciated!