“How far can Congress go ‘full jus sanguinis’ without redefining the term NBC?”
Under the Constitution, Congress has authority to define who is a citizen at birth and who is foreign. Is Congress always wise in how it uses its power? No.
I think, under original intent, Congress considered citizenship to be conferred naturally from parent to child at birth.
I think, under the 14th amendment, it appears Congress intended to also confer citizenship naturally by place of birth (within the US). However, their intent is not 100% clear.
“Where do you find the authority for the limits of NBC, if there is one?”
No one disputes that a child born on US soil, to two parents who are both US citizens at the time is a natural born citizen. Congress can not make someone born on foreign soil to two foreign parents a natural born citizen. I do not think Congress had the Constitutional authority prior to the 14th amendment to make children born in the US to two foreign parents a natural born citizen. The 14th amendment is what introduced Jus soli (though the term is absent).
The Jus soli legal theory derives from British common law rather than natural law. I do not think that was the founders’ intent as evidenced by 1790.
Again, what Congress can do and should do are two different things.
So, you see no constitutional impediment to pure jus sanguinis. Starting with a number of US citizens who move abroad, their children are NBC by jus sanguinis (current law), and if the naturalization act was amended to remove the US residency requirement of the citizen parent, the US could have an enclave of NBC's anywhere in the world, without ever living in the US. Unwise, but constitutional. You haven't identified any limit to this, not in natural law, not in Vattel (it's in there though, I see it), not in the constitution.
-- The 14th amendment is what introduced Jus soli --
The Art IV sec. 2 phrase "citizens of the several states" isn't a jus soli principle?
I know you see this as some sort of imposition, but could you please say, in just a few words (takes less than 20 characters), where you find jus sanguinis in the US constitution?