Basically, the vice president (elect) shall be acting president if the president elect fails to qualify. And if both are deemed ineligible, Congress determines who is president.
Using the VP elect would be rewarding the illegal behavior. It would be a bait and switch. The Constitution is clear about who can run and who can't run. He shouldn't have gotten past the primaries.
In all fairness, McCain was the only qualified person elected to the office. He got 48% of the votes while Oboma would be stripped of his. That makes McCain the automatic winner.
I’m not trying to argue, just interpret what the text says:
Section 3. If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President. If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President elect shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified.