Again, the plural can be read as more than one citizen who do not share offspring. Father of him, father of that other guy, your mother, my father, McCain's mother, GWB's mother, etc. If it meant 'fathers AND mothers who are BOTH citizens' they would have said that for clarity. And, again, at the time the mothers were not really factored in, women being a protected class with limited rights.
"Emer de Vattel's 'The Law of Nations,' upon which modern international law and political philosophy is built, has a little higher credibility than Dictionary.com."
Dictionary.com's credibility has thus far only been questioned only by those that don't like the dictionary definition of 'natural born.' Is Webster's credible? That is where dictionary.com gets the definition. Vattel's 'The Law of Nations' might have bearing if there wasn't already a body of US Law and case law on the subject that has different conclusions. Vattel's definition of 'natural born' is subsumed into the one supported by US Law - it is a subset, not mutually exclusive.