Hi Michael, Karsten and futagoza.

I don't think that Obj format is suited for handle vertex color data.

I think PLY format will do.

This a simple vertex colored sphere that I've done in 3D-Coat and painted then exported in PLY format : http://take.ms/F15Lh

This is the 3D-Coat screen capture :



And this is the same sphere imported into Blender with material that read the vertex color data :



Ho this can help.

Ciao!

Marco (mkdm)