What I can see in the information you’ve provided, the bake settings are wrong. You’re trying to bake full render into vertex colors. Baking mode should be vertex colors and “bake to vertex colors” should be unchecked.
Cycles doesn’t have a direct option to bake vertex colors on an image, and it’s not possible to bake a texture (image or procedural) to vertex colors with it.
I probably messed up my reply, leaving direct quotes out
I won’t help with that. Someone else of course could
The rest was information about the limited options for baking vertex colors in Cycles. Doesn’t bake colors to image directly, and can’t do it the other way around at all.
In Cycles you can plug the vertex colors into an emission shader (with the “Attribute” node) and then set the bake type to “emit”.
The name field in the “Attribute” node needs the name of you vertex color set.