I painted an group of objects in Blender using the Vertex Paint and was wondering if it would be possible to unwrap the mutliple materials(when the objects are joined together using Ctrl+J)? Do I have to paint it all over again?
You can bake Vertex colors to a texure.
Starting from a vertex color layer as in the picture below
you must unwrap your model (which may consist of one or multiple meshes) and create a new image to bake onto ("untitled" in my case)
Than go in the Bake panel of the Render tab and set Bake mode to Vertex color.
When the process is completed you should see something like the picture above.