Old post, but first on Google: I have a change ready that enables VertexColor GI. It will be exposed with 4 checkboxes in the Material, one of them enables scaling the diffuse color by VertexColor.RGB. So passing in LightmassReplace = (1,1,1,X) would give you pure vertex color GI. There are checkboxes to tint and scale the emissive as well (static emissive vertex color lighting). I’ll update this post when I have the Pull Request ready, I might need community support to push this faster since it’s a slightly bigger change. 
Example with Diffuse Boost = 10:
