If it's after the convention, the VP pick steps up, that's easy.
If it's before the convention, if Clinton has made a VP pick, it would go to an open convention, but the VP pick would have a stronger claim than Biden or Sanders. Hence, why she must make a VP pick ASAP.
Bernie Sanders would still have 1,831 released delegates who would be inclined to support him in an open convention. Why would the "claim" of a candidate who won 22 nominating contests be "weaker" than the VP pick of a candidate undergoing criminal proceedings?