how can i bake this vertex paint into texture?
You’re using your vertex color as texture coordinates? That’s unusual. It’s kind of what UV are for. I guess, yeah, they’re interchangeable.
What do you want to bake, the texture as looked up by your vertex color coordinates, or the actual coordinates represented by your vertex color?
Create a UV map, plug the value you want to bake into material output, create an image texture node and a blank image to go along with it (in sRGB mode, either way, IIRC vertex color lookups are treated as sRGB values), make sure that image is the active selected node in your node group, bake emission.
I think you misunderstood how Col is used. Connect the attrib node the material output directly, then bake “emission” , before that add a new texture in your node, and select it. You do not have to connect it to anything. The bake function will bake the resulting image to the selected image node.