That's my understanding as well.
But by the exception of both Vattel and Blackstone, the person would be a Natural Born citizen independent of the statute.
Do you have a cite of a Supreme Court decision, or even dicta from such a decision, to support this contention? What is the evidence supporting such an interpretation, other than the existence of an exception under both Vattel and Blackstone?
The problem is that Natural Born Citizenship is only a factor in eligibility for the Office of President. No such case has ever come before the Court, so they have had no cause to rule on the issue. There is dicta on the "of two citizens born in the country" part of Vattel's criteria, (cited in Wong Kim Ark, but dicta even in the original case) I know of none specifically on the "in the armies" provision of Vattel.