Clark leaked because he thought it would help his campaign. He actually thought he could win.
I believe Clinton convinced him of that, to get him to run in the first place.
Why? Not to get a VP nod. I think he's a seat-filler for the Hillary campaign, if she decides to run.
IMO the Clintons lay plans for several different strategies to achieve their goals, choosing the most likely and discarding the rest at the last minute. In this scenario, Hillary announces at the hopelessly deadlocked convention in July. There won't be time to build a national network of campaign staffers. To hit the ground running, the Clark machine becomes the Clinton machine overnight. Surpise! (to everyone, including Clark)