According to the man who wrote the first Constitutional legal treatise [which got him an appointment to the Virginia District Court by James Madison] Vattel was used for internal operation as well. Here he connects Vattel directly to the 10th Amendment:
And because this principle was supposed not to have been expressed with sufficient precision, and certainty, an amendatory article was proposed, adopted, and ratified; whereby it is expressly declared, that, "the powers not delegated to the United States by the constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people." This article is, indeed, nothing more than an express recognition of the law of nations; for Vattel informs us, "that several sovereign, and independent states may unite themselves together by a perpetual confederacy, without each in particular ceasing to be a perfect state. They will form together a federal republic: the deliberations in common will offer no violence to the sovereignty of each member, though they may in certain respects put some constraint on the exercise of it, in virtue of voluntary engagements."[57]
George Tucker, View of the Constitution of the United States with Selected Writings [1803]
“This article is, indeed, nothing more than an express recognition of the law of nations...”
Law of Nations = International Law. In your case, referencing what happens if “sovereign, and independent states” - state being another word for country - decide to join together in a “perpetual confederacy”.
I wish the states of the US were sufficiently sovereign so that “the deliberations in common will offer no violence to the sovereignty of each member”! Unhappily, the Civil War and the 14th Amendment destroyed that concept.
Still, your quote was using Vattel as an authority on International Law, which he was. He was NOT an expert on American law (which didn’t exist in 1757), nor was he very knowledgeable on English law.