Vertex Colour information is great and all, but not if you have to be in Vertex Paint mode to actually see it. If only there was a way to display it in Blender’s Object Mode, ey? Thankfully there is!
Head over to the Viewport Shading options (that little drop-down menu next to the Viewport Render styles), then switch from the default Material Color to Attribute Color. Now you can see all Vertex Paint animation.