If Romney had picked Petraeus for VP

[minionofmidas]

You're discussing impossible, because Petraeus, as CIA Director, was legally barred from getting involved in political campaigns, much more being a candidate himself.

He would have had to resign as such.

The big question would be why should he accept such a nom. I very much doubt he voted for Romney. People in high appointed office, like close family of candidates, almost always vote for their team even if their political views are not fully aligned.