If that happens, I don't see how the Democrat Party can convince Bernie's supporters, who are absolutely fanatical about their candidate, to fall in line and support the ticket. If they feel that Bernie got screwed out of the nomination, they'll walk away from the party. If Hillary isn't the nominee, I think the DNC will have no choice but to roll the dice with Bernie and hope for the best.
That’s why Warren would have to be on the ticket.
Or, maybe they go full pander: Hillary names Warren as her veep then steps down—and Warren names a Castro brother or Deval Patrick as her veep.