The Constitution and the rule of the common law.
What law do you rely on for your claim that it takes birth on US soil plus two citizen parents for a person to be a natural born citizen?
And don't say Vattel. Because Vattel's opinion is not in itself the law of the United States, unless the courts say it is. Vattel is an opinion of a foreign writer on the law of nations. Other writers on the law of nations had different opinions. And citizenship, the relationship between the individual member of our nation and our national government and community, was never a matter of international law. It was a matter of our own domestic law.
Where in the Constitution is a foreign born child of parental US citizenship declared to be a “natural born citizen” or a “citizen”?
In 1790 the husband and wife were one person in law, hence the “two citizen parent” claim.
Between 1790-1795 is the only time a class of persons was explicitly declared in law as “natural born citizen” and such citizenship was dependent upon parental US citizenship.
Are we to conclude that subsequent to 1795 there were no further “natural born citizens”? Or are we to conclude that the other children born with parental US citizenship - you know, those who were not “born beyond Sea, or out of the limits of the United States”, ie in country - are “natural born citizens”? Or are those other children born with parental US citizenship within the United States something other than “natural born citizens”? Who then are the post 1795 natural born citizens?
A child born in country to citizen parent has always been a citizen, without operation of law, this has never been doubted. These are natural born citizens.