I will point out something else to you. Prior to July 4, 1776, everyone was a British SUBJECT. Usage of the word "Citizen" to replace it didn't come about until around this time.
If you look up usage of the Word "Citizen" prior to 1776, you find it used to describe a member of a City. Search Shakespeare, or other writings of the time, and it is seldom if ever used to describe a member of a Nation.
Now where did this change in meaning come from? Who was using the word to describe the members of a nation around this time?
I'll give you a hint.
Le droit des gens, et les devoirs des citoyens, ou principes de la loi naturelle, appliqués à la conduite des affaires des nations et des souverains
What i'm getting at is that the usage of the word "Citizen" has Vattel's fingerprints all over it. That Vattel used it is the reason we also happen to use it... Now.