The Word "Citizen" itself clearly demonstrates that Jefferson copied it's usage from Vattel. In 1776, the normal word to describe someone as a member of a nation state was "Subject."
At the time, no one used the word "Citizen" to mean member of a Nation State except for Switzerland, a Republic since 1370.
No. You can also find the word "citoyen" (citizen) used by French Enlightenment thinkers to mean "a member of a free society with many families, who shares in the rights of this society, and who benefits from these freedoms." It did not necessarily refer to a resident of a city. And this was before Vattel published his book.
If you are referring to "Rousseau" or "Burlamaqui", they were both Swiss, not French.
So far as I know, Rousseau was the most influential French speaking Philosopher of the time period, and he was from Switzerland, not France.
If you have an example of someone else using the word "Citoyen" that does not have roots in Switzerland, I would like to know about it.
Switzerland was a Confederacy that was originally made up of 6 Cities, and so in Switzerland, the term "City Dweller" came to mean being part of that coalition of cities. The usage of the word "Citoyen" to describe a member of their Republic started with their "Priest's Charter" (Swiss confederacy founding document) in 1370.
Etymology on the word "Citizen" to mean member of a nation/state traces it's origin to the late 14th century. 1370 appears to be when this practice began.
It stands to reason that when a Union made up of differing Cities began, it would have been reasonable to start referring to "City Denizens" as the term to describe the members of their Nation.