If you look at 1790's immigration law (United States Naturalization Law of March 26, 1790), that early congress defined a person born of citizen parents, outside the United States, a "natural born citizen." Yes, that phrase. That right there describes George Romney and is proof positive of founding thought on the matter of whether a "natural born citizen" had to be both Jus Soli and Jus Sanguinis born in spite of birther's treatment of Vittal as holy writ on the matter.
At the time of George Romney's birth Mexico specifically outlawed birthright citizenship for aliens. There was no Jus Soli in Mexico and both of George Romney's parents were only ever American citizens. He was never a Mexican citizen.
End of story.
Facts like these, or expert opinion (like Mark Levin's on Rubio) won't sway birthers from their pet theories.
“...The Naturalization Act of 1790...” was rescinded before 1800.