It is a fallacy to conclude that the failure to mention something is the equivalent of rejecting it. In looking up the link for the Breckenridge Long article, I happened across some commentary on it.
The commentary noted that Breckenridge Long had failed to mention Minor v Happersett, and therefore the Minor v Happersett case is irrelevant or trivial or something.
As I was reading through the article, I thought to myself, whoever wrote this has less reasoning skills than a monkey with their hand caught in a jar. I looked up the author's name. It was "Squeeke Frome", one of the silliest people i've ever encountered on Free Republic. (Now banned, and for good reason. She was a crackpot of the first order. )
The point is, you can't read too much in the absence of information. A failure to mention something does not preclude it's involvement.
As I mentioned before regarding language, the important aspect is the principle involved, not the language or in this case, the name of the guy who first thought of it. (Actually, Aristotle beat Vattel by thousands of years.)
Several other court cases use the Vattel definition but do not reference the name Vattel, or "law of nations." If it was regarded as American Common law (and Breckenridge Long seemed to think it was) then he didn't need to note it's origin.
Breckenridge Long asserts the citizenship principle of Vattel, whether he mentions the name or not.
Same thing with Madison. (See case of James McClure.)
Long was a Democrat, a partisan with political ambitions of his own. His motives for raising questions about the eligibility of the Republican nominee in an election year are probably no more complicated than that. But, then, maybe partisanship is the only reason that anyone has ever become interested in this NBC issue. ;-)
Believing as I do that the NBC standard is a constitutional standard to be applied by constitutionally-selected electors free from any interference by our national judicial or legislative branches, I don't look to the Congress or to the federal courts for authoritative guidance as to the precise meaning of the standard. The Supreme Court has never even considered overruling the decisions of electors. For good reasons, the justices limit their participation to just showing up for the inaugural celebrations and helping with the oaths of office.