FWIW, your definitions are of the English word indigenous. While I assume this definition is quite similar to the equivalent word in French, there may be shades of meaning that vary.
Since we’re dealing with shades of meaning here, that could be pretty important.
I agree with you that using Vattel as a primary argument for the legal definition of NBC is a pretty weak reed.
I have yet to see any convincing argument that NBC means “citizen at birth” as opposed to “naturalized citizen.” If so, its specific parameters would change as the definition of citizen at birth changed, notably with the 14th Amendment.
Should read "I have yet to see any convincing argument that NBC means anything other than “citizen at birth” as opposed to “naturalized citizen.”