Hi Grafix -
Thank you for the new asset. From looking at it, it appears that the Vertex Painting is working as expected, though there are sections of your model where you do not have even vertex placement which will affect how the model takes vertex painting.
Here is a picture showing Vertex Coloring and a Blue Wireframe of the model to show you the difference I am see in the even vertex placement (Left) versus the uneven placement (Right).
Thank You
Eric Ketchum