John McCain was born in what was a U.S. unincorporated territory. His citizenship derives from his parents, both U.S. citizens, and from the statutory U.S. citizenship granted by Congress. He is not, however, a Natural born Citizen and has never been eligible to be President.
John McCain has both statutory and declaratory U.S. citizenship. He is as qualified to be a natural born citizen as any of those were born abroad who were declared to be so by Congress in the late 1790s.
If Vattel is the authority cited for the natural born citizen requirement as written into the constitution, then Vattel is also the authority for children born to citizens serving their country abroad.