I have never heard of anyone who has actually researched this topic suggesting that Grandparents had to be American Citizens. Only those people who have misunderstood the topic have opined such a thing, and they are a tiny minority, and unworthy of mention.
Flexibility: The flexibility of a Constitutional provision concerns the degree to which the provision forces a future interpreter to make principled choices when applying the provision.
Once again I must point out that strict constructionists do not favor flexibility in interpretation. As Thomas Jefferson said:
The two enemies of the people are criminals and government, so let us tie the second down with the chains of the Constitution so the second will not become the legalized version of the first.
Permitting Congress to modify the meaning of "Natural born citizen" is tantamount to weakening the chains, and may render the intended protection of the article inoperative.
My Interpretation: My interpretation of the NBC clause to require only citizenship at birth indicates that I interpret the clause more broadly than you do.
No argument there. Foreign born to a Foreign Father is a very broad interpretation indeed, and one that could not exist at all but for a law Congress passed in 1934. Whatever it is, it isn't "Natural" as the founders saw it.
My interpretation is (I believe) rationally based upon my firm belief that ordinary people now and ordinary people in the 18th century would find that interpretation to be a more natural interpretation of the clause.
You suffer from a disconnected Zeitgeist. It might feel natural to modern minds, but it wouldn't have been regarded as "natural" at all in 1787. Born in a Foreign country to a Foreign father? By every understanding of the law (even the English Common Law) in 1787, that would have made you anything but an American.
Your Interpretation: Your more narrow interpretation is (I believe) rationally based upon your research and discovery indicating that some of the most influential folks in the founding generation looked to Vattel for guidance as to theories of citizenship (coupled with your English translation of his French text).
Not just research but logic. The dominant interpretation yields paradoxical history and nonsensical results in the Present. I'm sure I don't need to point out to you the steadily worsening problems of Anchor Babies and Birth Tourism, and I have repeatedly pointed out that the native born Children of British Loyalists were recognized by both sides as British, Slaves didn't become citizens until 1868 after they passed a SPECIAL LAW (14th Amendment) Recognizing them as such, and Indians did not become citizens until 1924 after another such special law was passed naturalizing them en masse.
The popular interpretation is simply incompatible with these actual historical events. The popular interpretation requires that the founders (and framers of the 14th) have no inkling of the abuses which would follow, and therefore they were short-sighted and/or foolish.
Logic dictates otherwise, and both George Will and Ann Coulter have written articles supporting this very conclusion.
As I've said before, I'm not saying your interpretation is more right or wrong than mine. It's just more narrow.
It better accomplish the purpose for which Article II was intended, as outlined in Jay's letter and as Written of in the Federalist Papers. You interpretation requires that Aldo Mario Bellei, who was stripped of his Citizenship, could have been President.