Absolutely. This a simple choice case. Either he is NBC due to birth in America from American mother or NBC must come from two American citizen parents. Court could rule either way, but I would say that tie goes to the runner and obama would win.
It is inconceivable - literally beyond even imagining - that a court would a) adopt the Vattel definition and then b) impose it retroactively on a person who was elected President with 69 million votes.
It won't happen, it never could happen, and if it happened to MY guy I would say it was wrong, beyond the scope of the court's power, and a cause for armed resistance.
Now, I think it would be wise for Congress to write a definition of NBC into the US Code, but if Congress wrote a law adopting Vattel going forward, I think it could never be made to apply to Obama, both on ex post facto and Bill of Attainder grounds.