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.
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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.
Hey, I’m not the one drawing a legalistic distinction between the idea of subject and citizen.
The concepts in common law that applied to subjects were transferred to the United States legal system under the term citizens, since our country has no subjects.
The idea that there is a distinction is due to a ludicrous claim that all common law references to subjects and their allegiance are not transferrable to the United States, since we don’t have subjects.