Works for me, with current trunk (+ CW experimental, but there's no change related to vertex colors in there). There is a difference whether or not you have a second vertex color layer named "Alpha" (even if it's fully white, translating to opaque), as mentioned in
this thread, have you tried both? If I understand my messy research notes from back then correctly, you wouldn't see anything of your vertex colors if you don't have an Alpha layer (the kLiteVtxNonPreshaded flag is not set) and your material ambient component is zero.