From a comment by Ripper McCord at TPM:

Appearing on Late Night with Conan O’Brien in July, John McCain was reminded that [he] often tells his audiences the vice president has only two duties: Breaking tie votes in the Senate and inquiring daily about the health of the president.
“That job will be very, very important in my case,” replied McCain, who turned 72 today.

And this was the most qualified candidate he could come up with…?
UPDATE: Tom at The Inverse Square Blog runs the numbers…