While it doesn't consider what the OED has to say about the word citizen, I just looked and the OED broadens the definition specifically for the US. They do give Johnson's definition first, but the second they give the broader US definition with the first usage example from 1538.
ML/NJ
I looked at your thread. Indeed, I had responded to it back in January. I am commenter number 5. I didn't see a definition for the word "Citizen" in there anywhere. Do you have a link to the Oxford dictionary prior to 1770 so that we can see how they defined the word "Citizen" back in 1770? (Before Vattel's influence could have any effect.)