The Sanders supporters will split their votes. I mean, if your rationale was applied evenly, and the ticket was Hillary/Sanders, it would read "I think he should ignore Hillary. Lots of Sanders supporters love her and if he wants the crossover vote, targeting her will undermine that." The approach he is taking is the right one, target each one as an individual. The chips fall where they fall.
It would be wonderful if we could install Burney as the Green Party nominee. But I suspect he’d be an Arkancide victim if he did it...
It’s hardly equivalent if we’re talking about running mates. Warren isn’t anybody’s running mate. If she somehow ends up as Hillary’s VP pick (she won’t - they do not like each other AT ALL), then of course he should go after her.