I'm sure it was, or it would have been more clearly defined. However, there's no definitive proof, anywhere, that de Vattel's views on citizenship (two parents, both citizens) was the common understanding of the phrase.
The fact that jus soli is the common interpretation of the term today is not a reinterpretation, it's acknowledgment of the modern reality.
How high do you have that bar raised? I would call this pretty definitive:
Benjamin Franklin’s (a signer of our Constitution) letter to Charles W.F. Dumas, December 1775
“I am much obliged by the kind present you have made us of your edition of Vattel. It came to us in good season, when the circumstances of a rising state make it necessary frequently to consult the Law of Nations. Accordingly, that copy which I kept (after depositing one in our own public library here, and send the other to the College of Massachusetts Bay, as you directed) has been continually in the hands of the members of our congress, now sitting, who are much pleased with your notes and preface, and have entertained a high and just esteem for their author”?
John Yinger is Professor of Economics and Public Administration at the Maxwell School of Citizenship and Public Affairs, Syracuse University, and the father of two adoptive children, one of whom, even when old enough, will not be eligible to be President.
The term "natural born" citizen has a long history in British common law.(38) In fact, a law passed in 1677 law says that "natural born" citizens include people born overseas to British citizens. This usage was undoubtedly known to John Jay, who had children born overseas while he was serving as a diplomat.(39) It also appears to have been employed by the members of the first Congress, who included many of the people who had participated in the Constitutional Convention. To be specific, The Naturalization Act of 1790, which was passed by this Congress, declared "And the children of citizens of the United States, that may be born beyond the sea, or out of the limits of the United States, shall be considered as natural born citizens; Provided, that the right of citizenship shall not descend to persons whose fathers have never been resident of the United States."(40)
In other words, they were drawing on the English legal tradition, which protected allegiance to the king by conferring citizenship on all children "whose fathers were natural-born subjects," regardless of where the children were born.(44) Thus, according to Morse, "the framers thought it wise, in view of the probable influx of European immigration, to provide that the President should at least be the child of citizens owing allegiance to the United States at the time of his birth."(45)
Morse also emphasizes the difference between the terms "native-born" and "natural-born." The dictionary, which follows the English precedents, defines "native-born" as "belonging to or associated with a particular place (as a country) by birth therein" and "natural-born" as "having a specified status or character by birth."(47) If the Founders had not wanted an expansive definition of citizenship, Morse writes, "it would only have been necessary to say, 'no person, except a native-born citizen.'"(48)