Mav: first of all, RFK never backed Gene. They hated each other with a passion (among other things, McCarthy implied that RFK courted death by wearing a skullcap in Oregon), and Lowenstein & Co. approached Bobby in the fall of 1967 to be their standard-bearer. He refused for a variety of reasons, mainly because he wasn't sure the enterprise would succeed. RFK became known as the original "Hamlet on the Hudson" (unusual for him), and by New Hampshire, RFK basically told Gene to "Get the fuck out of my way, you were only the pathfinder". He didn't, as we know. Daley would never allow McCarthy to be nominated, and since 75% of the delegates were boss-controlled, I call ASB on a Gene McCarthy nomination. Both men would rather nominate Hubert than each other, and McCarthy even said so in an interview just before California. A VP McCarthy would be more plausible in 1964, when LBJ wants a Northeastern, relatively young Catholic whose name is not Robert F. Kennedy.
EDIT: If you want to unite the Democratic Party, you can have RFK as VP- if only for fundraising reasons, since he was the only Democrat who could bring in vast amounts of cash and compete with Nixon or Rocky in that field. Since they work well together, it keeps everyone happy and lines Bobby up for 1976 whether HHH wins reelection or not.