... and, we're not attempting to prove the practice in England. There was a rather profound rift, if you'll recall, resulting in the Revolution. Claims of "perpetual allegiance" were mocked in the immediate era leading up to the Constitutional Convention, mocked as "perpetual nonsense," referred to as an absurd barbarism.
It was mocked because it was detested. It was detested because ... why, mlo? Are you incapable of grasping that being deemed a natural-born subject by England was at the root of the problem, and not the solution to the problem in any way, under a new Constitutional Republic?
Curiously, you seem to think that Vattel having compiled two volumes of Natural Law known as "The Law Of Nations" somehow makes his effort less applicable, because it covered both national and international law. Well, at least he delved into something outside of the very English monarchy that our Founders risked their lives to revolt against, unlike Blackstone.
Yes, the 500+ references to princes in "The Law of Nations" show that Vattel was willing to entertain various flavors of monarchies, not just the "very English" variety. The man talked a *lot* about the role of a monarch in relationship to his nation, and the role of a people to their monarch.
How the Founders could have relied so heavily on Vattel, yet so roundly rejected his copious writings about sovereigns and princes being the default and most natural governmental arrangement, will just have to be a mystery.