Keaton became the second person to leave the Vice Presidency in the same presidential term. Mac had left the vice presidency to become President while Keaton resigned.
In real life the vice-presidency did become vacant twice in the same presidential term during the 1973-77 term. Congress had first approved Gerald R. Ford who ultimately became President, and then Nelson A Rockefeller who served out the rest of the term.
Unlike Ford and Rockefeller, Keaton being a Congress approved Vice President ended the vice-presidency without having to become President nor serving out the remainder of the term.