Go back and read the Constitution, especially the 12th Amendment. If Obama is declared ineligible, then the Electoral College will not be able to pick a candidate, because no one will have a majority. Then it goes to the House which MUST CHOOSE from the 3 remaining candidates receiving the most electoral votes. Only McCain is in this category (Biden is NOT a candidate for President), since he is the only oneother than Obama to gain electoral votes, and thus it would fall to him.
In the end, we’d have McCain and Biden, Prez and VP.
If he were disqualified before the electors vote, the electors could choose anyone from what I understand - they would likely choose Hillary over Biden. Biden might still be VP.