NBC is indeed a term not defined in the Constitution. Neither are: Militia, arms, people, welfare, commerce, defense, or State defined. For those we must look at the references which show the definitions of the day, ones to which the Founders had access, just as someone today might look in a Dictionary to find the definition of "is".
When one looks at the definition in Vattel's and regards that Obama was a British Subject due to his Father's citizenship status, even the claim of dual citizenship is not enough to pass muster as a Natural Born Citizen, without conflicting loyalties.
Respectfully, that is your opinion. That is all it will be until a Court validates it. With sympathies for your frustration, I do not believe any Court will ever do so.