It seems like vertex color data on meshes aren't taken into account when lightmass calculates its lighting.
Simple mat that uses the red vertex channel and a mask to blend between two colors. (Extreme green and pink for easy viewing
)

Result after vertex painting and baking -

As you can see, after vertex painting green is the dominant color on the mesh yet it still bounces around just the pink color.
After switching around the green and pink constants in the lerp node -

The opposite occurs, making it seem like lightmass only takes into account of the color / texture that is connected into the b input of the lerp node and ignores all vertex painting.
Simple mat that uses the red vertex channel and a mask to blend between two colors. (Extreme green and pink for easy viewing

Result after vertex painting and baking -
As you can see, after vertex painting green is the dominant color on the mesh yet it still bounces around just the pink color.
After switching around the green and pink constants in the lerp node -
The opposite occurs, making it seem like lightmass only takes into account of the color / texture that is connected into the b input of the lerp node and ignores all vertex painting.
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