And yes, I recognize Vattel’s influence on modern politics and no, I’m not trying to downplay that. But seriously, basing a 21st century-American law on two sentences of a Swiss book from 17something or other?
What “21st-century American law” are you referring to exactly? Be specific.
The only law at issue here as far as I can tell is the US Constitution “from 17 something or other.”
When you're trying to understand the intent behind the U.S. Constitution, also written in "17something or other", a legal book from the same period seems like a pretty good place to start :-)
Applying modern definitions to historical language without understanding the original use will likely result in misconstruing the meaning; each generation tends to modify meanings to suit their own understanding or viewpoint. "Gay" used to mean "merry"; a "hack" used to refer to a hired horse; and that does not even scratch the surface of slang stuff like "bad" and "sick" that can mean what they say, or exactly the opposite.
Written works make no sense without understanding them in the context of the time they were written.