Not just Vattel. There is basis for it in English law as well. Here are two different English Law Authorities.
Matthew Bacon, A New Abridgement of the Law, Vol 1, 1736
The common law common-placed: By Giles Jacob, 1726
And i'll throw this in for good measure. (Look to the very end.)
City Liberties: Or, The Rights and Privileges of Freemen. Being a Concise Abridgment of All the Laws, Charters, By-laws, and Customs of London. 1732
And then, of course, there's just plain old common sense which informs any man that it is stupid to use the authority of Feudal Land-Bondage to decide American Citizenship.
Can’t use “Scottish Law” anymore, Arlen Spector is no longer with us.