My hunch is that Clinton has his mind on a third term for the Presidency: He will run on the top of the ticket, with Zer0 on the bottom. Biden can work for Hillary in the State Department.
Scary. But fortunately we have the 22d Amendment: "No person shall be elected to the office of the President more than twice ... ." (Nor could Bill Clinton run as VP. Anyone ineligible to be President can be Vice President. That's per the final sentence of the 12th Amendment.)