Charles Curtis was not born in the United States. He was born in the territory of Kansas.
This is the same argument that was tried against Barry Goldwater as he was born in the territory of Arizona.
According to the strict definitions that have been put down as the line of legitimacy, neither Curtis nor Goldwater (nor McCain for that matter) would be a “Natural Born Citizen” according to Vattel.
This argument fails on all accounts, everytime it’s brought up.
There are two ways (and only two, according to the words of the Supreme Court above) to be a citizen. You are either born a citizen, or you are naturalized a citizen.
If you are born a citizen, you are a “natural born citizen”. Period.
The territory of Kansas was part of the United States, just because it had not reached statehodd did not remove it from the US.