In order to demonstrate my problem, I created a simple scene with a cube and I used Vertex Paint to color one corner.
When I export to COLLADA, I make sure that "Include Material Textures" is checked.
The vertex colors are not saved properly. I've verified this by importing the COLLADA file back into a new Blender file and also by importing it into MeshLab, Sketchup, Photoshop and 3DS Max, none of which showed colors.
I know the Stanford PLY format also supports vertex colors (and it worked well for this same cube), but the actual scene I'm dealing with has thousands of little complex objects and PLY can only export one object at a time. I could write a script to export each object, but I'd rather get into one file.
(using Blender 2.69, Linux 64-bit, any more info needed?)