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.
Or Samuel von Pufendorf's 1673 De officio hominis et civis juxta legem naturalem libri duo (On The Duty of Man and Citizen According to the Natural Law Two Books)
"The duration of the citizens' particular duty is, so long as they fill the office from which the duty springs; and when they leave the same, the latter too expires. In the same way the general duty lasts as long as they are citizens. But they cease to be citizens, if they leave with the express or tacit consent of the state, and fix the abiding-place of their fortunes elsewhere; or if for some crime they are exiled and deprived of the right of citizenship; or if they have been overpowered by the enemy, and compelled to submit to the rule of the victor."
Here is Cunningham's dicitionary
https://archive.org/stream/newcompletelawdi01cunn#page/n504/mode/1up
BTW, During the debates over the 1795 Naturalization Act, Congressman Giles proposed an amendment that would require immigrants with titles of nobility to renounce the title before becoming naturalized as an U.S. citizen. Mr Hillhouse of Connecticut believing the amendment did not go far enough and proposed the following hypothetical case;
"If we pass the present amendment, the construction must be, that an alien, after residing in this country, abjuring his allegiance to his own, offering to become a citizen of, and taking the oath of fidelity to, the United States, is in the possession of the rights of a privileged order to which he may have belonged; and further that their rights are hereditary, unless he shall, agreeably to the amendment, come forward and renounce them. But what will be the consequences of him not renouncing? Most clearly that he retains and possesses them. A nobleman, then, may come to the United States, marry, purchase lands, and enjoy every other right of a citizen, except of electing and being elected to office. His children, being natural born citizens, will enjoy, by inheritance, his title, and all the rights of his nobility and a privileged order he possessed, an idea which ought not, either explicitly or impliedly, to be admitted." Annals of Congress, House of Representatives, 3rd Congress 2nd Session, January 2nd, 1795 page 1046
How can a nobleman come to the United States, not become a citizen but have children who are natural born citizens?