The constitution says what it says - it was thoughtfully composed. One can look to the Articles of Confederation as the direct predecessor. Citizenship in the union followed citizenship in the states, and under the constitution, states were precluded from naturalizing people into the union - that was left for Congress.
I think you are right on the history or usage of the word "citizens." The founders inverted the power pyramid, and "subjects" would have been anathema to them. The subjects became the king, and the king was subservient to the subjects.
Voeltz v Cruz Motion to Dismiss on Scribd. https://www.scribd.com/book/297538440
While pondering this issue earlier today, I got to wondering if there was some way to better establish that our usage of the word "citizen" derives specifically from Vattel, or does it come from some other source.
On a lark, I decided to see if the word "citizen" was used in any of the law dictionaries in usage in the early 1770s. I found Giles Jacob's law dictionary from the period. A search for the word "citizen" finds nothing listed in it.
Clearly Giles Jacob's law dictionary is not the source of this word.
I looked in Matthew Bacon, A New Abridgement of the Law, Vol 1, 1736), and the word "citizen" does not appear in it anywhere.
I tried to look in Timothy Cunningham. A New and Complete Law-Dictionary, or, General Abridgement of the Law. (1765) But could find no online copy of it.
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I am thinking that if it can be established that the word "citizen" was relatively obscure, and either wholly or mostly unfamiliar to English Law, then this becomes powerful evidence that the source of that word usage is likely Vattel, and that the very word "citizen" is proof that the Vattel definition was intended to apply.
If it was rarely used in English up to that point, then this strongly implies that it's source must be someplace else.