“For the same reasons also, children born out of the country, in the armies of the state, or in the house of its minister at a foreign court, are reputed born in the country; for a citizen who is absent with his family, on the service of the state, but still dependent on it, and subject to its jurisdiction, cannot be considered as having quitted its territory.”
According to Vattel the children of citizen parents born overseas would have the same conditions of birth than if the child had not been born overseas.
That is why claiming the children of U.S. citizens born overseas to U.S. troops (like McCain) are not natural born IS abandoning Vattel.
And the children of foreign parents born in our nation would not be citizens at all if we followed Vattel. Vattel did not think granting them citizenship of a different type at birth was the proper answer under natural law - he opined that they shouldn't be granted citizenship AT ALL.