Unfortunately for your theory, when Vattel wrote there were almost no citizens (in the Republican sense you mean) in Europe. Almost all Europeans were the subjects of kings far more absolute than the kings of England.
Use of the term citizen in the sense you mean developed during the French Revolution. Vattel had been dead for a long time by then.
So now citizen doesn't mean citizen?
So what did it mean?
Citizens obviously did not have same sorts of rights as modern ones do, but they were still different from subjects, who owed personal and permanent (in theory) feality to the monarch.
Unfortunately for your theory, when Vattel wrote there were almost no citizens (in the Republican sense you mean) in Europe. Almost all Europeans were the subjects of kings far more absolute than the kings of England.
Use of the term citizen in the sense you mean developed during the French Revolution. Vattel had been dead for a long time by then.
------------------------------------------------------------
I believe Vattel was describing concepts of natural law as he saw it, and not a particular government or society found exclusively in Europe at the time in which he lived. His work built upon previous writers of natural law...going all the way back to the ancient Greeks whose citizens had a legal right to participate in the affairs of the state.
Where did the Statue of Liberty come from and why, Sherman Logan?