If the Dems can dig up enough dirt on Sarah so that McC would have to replace her, either politically of morally, then Obama could dump Biden in favor of Billary while the public is busy. But it would have to appear to be fair and reasonable in the public eye.
Biden had a Brain Tumor removed in the past, and if the Dems want to make enough Hay about that fact in favor of possibly winning with Billary, then it will be done.
Biden is safe as long as Sarah is in. For Obama to replace Biden at this point for no apparent reason would be absolutley STUPID, and an event that would throw the election to McC for sure.
For my part, I believe there should be an investigation as to whether Obama is a bonafide Natural Born Citizen. He was born in Kenya, and from what I hear he and his Mother didn't fall under the law at the time regarding U.S. citizenship.
Not until much later was the law changed, and he would be a U.S. citizen now under the same circumstances at birth. But that wasn't the law when he was born.