His father was born in Mexico, but his parents, Mitt's paternal grandparents, were both US citizens. Thus his father was born a citizen, assuming he took the proper follow up actions required at that time. Mitt was born in the US, of parents who were citizens, even though his father was not a natural born citizen. Thus Mitt is a natural born citizen.
It can be argued, with reference to the definitions of "Vattel" in "The Law of Nature", that McCain was born when his father was serving the country, even though out of it and so was considered "born in the country", even if he was born in Pananma, which has not been proven to my satisfaction. I didn't vote for him the primary, but the facts are what they are on the issue.
§ 212. Citizens and natives.
The citizens are the members of the civil society; bound to this society by certain duties, and subject to its authority, they equally participate in its advantages. The natives, or natural-born citizens, are those born in the country, of parents who are citizens.
§ 217. Children born in the armies of the state.
For the same reasons also, children born out of the country, in the armies of the state, or in the house of its minister at a foreign court, are reputed born in the country; for a citizen who is absent with his family, on the service of the state, but still dependent on it, and subject to its jurisdiction, cannot be considered as having quitted its territory.
If you are going to use Vattel's definition for NBC, you really ought to use the special case qualifications too.
There has been no evidence that George Romney ever took the proper follow up actions. That was part of the problem back in '64 when he was a POTUS candidate. There were rumblings about his eligibility back then but he quicky dropped out of the race so the questions were also dropped.
Mitt has been shoved down our throats for a reason. That reason is because the GOP is cya-ing over allowing the Obama presidency stand. Once there's a GOP ineligible president then "they" have won. That old outdated Constitution will finally be shredded and it's welcome to the New Amerika.