That is irrelevant. George Romney (Mitt's dad), was a U.S. citizen at Mitt's birth.
The real question is Marco Rubio. Under this definition, he wouldn't be a NBC because his parents weren't U.S. citizens at Marco's birth.
Nor would Bobby Jindal, Nikki Haley, nor my husband be natural born citizens.
If we accept Rubio for Vice President we play into the hands of Obama by accepting another non NBC .
Thereby legitimising Obama.
By Mexico law, George Romney was born and was forever a Mexican citizen. No records of his naturalization as a US citizen were produced back in 1964 and 1968 when the issue was discussed during his primary races for president. He lost the primaries, and the subject was dropped.