Edit: For all viewers which can have similar problem - if your addon is importing vertex color but you don't see second or another one then you should make sure that this script even has these kind of feature.
I am working on *.dff files imported from game to blender so I can modify it and export back. Everything is working perfectly except I did not think about night in the game. Default files from game have both, night and day vertex color (in one file as one object) but I can display only day vertex color as default in Blender. I mean, I did not see any way to get to into second one. Menu -> Data -> Vertex Color -> shows one col. Is there a way or it depends on export/import .dff script?