A Vice Presidential candidate is usually chosen because he has a constituency in a major state.
No one even knows what state Cain is from. He has no constituency. Plus, he is a blowhard crackpot. If you were president, would YOU want to this annoying old gasbag sound off all the time?
I am reminded of the Seinfeld episode in which the real Cosmo Kramer fed pork and beans to the carriage horse when I think oif Cain as a VP..