Aw Geez! Did you really expect him to say POTUS when the question was "What would you do if you were VICE PRESIDENT OF THE US"?
Gingrich laughed a lot, and then didn’t answer the question. Gingrich asked Cain a serious question that would allow Cain to explain to the audience what he would bring to the presidency. Cain used his question to Gingrich to make a joke. There was no way as a candidate for President to answer that question.
I noticed it more on the 2nd time through — it was like Cain was pretending he was doing a job interview with Gingrich for the VP slot. Gingrich decided to take it as a humorous question, and ducked it with his humorous response. He NEVER answered the question seriously — because no Presidential candidate would.