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.
Yes, take things in context. So let’s understand that Vattel’s work was primarily concerned with established European lands such as Switzerland, nations with long-established populations and cultures living in close proximity to other nations with long-established populations and cultures. He wasn’t necessarily envisioning overseas colonies whose population was almost entirely fueled by immigration becoming nations of their own.