Hillary's main worry is that Bernie's supporters won't turn out and vote for her in the general election. This means that she may pick Bernie as her VP, or someone equally far to the left to please Bernie's constituents.
Having Bernie in place of Hillary would have pluses and minuses. The plus is that he isn't the crooked sleaze that Hillary is. The potential minus is that he's further to the Left on many issues (he's a socialist, she's a crony capitalist). Of course, we could get the worst of both worlds if Hillary picks Elizabeth Warren.
Yep, Warren was who I was thinking of.