Well, vertex color is one of many "attributes" you can "maintain" for a vertex.
It looks like in your picture you either have to much emissive for the object - OR the Vertex color isn't being honored when you load the model.
You can try changing the emissive so you're colors aren't "washed out" or try setting object light to zero (might not be a good thing but might be what you want)
Otherwise - you might try saving the model in a few different 3d formats supported by DarkGDK and see if they give different results and take the best one that works for you.
When you are making a model from scratch in memory - controlling the FVF formats is easier - as you have total control - however trying to load a model you are bound by the limitations of DarkGDK.
DarkGDK might not be a limiting factor here - and I think you'll need to experiment a bit.