It's tough because Bernie isn't going to compromise on his beliefs to play the #2 role very well. He's not going to do well sidestepping questions about Clinton that he disagrees with her on. That gives the GOP a target to exploit in debates.
It won't be easy, but if Hillary wants to win, she has to bring Bernie's supporters into the fold. He made an interesting remark during his Super Tuesday speech. "If you ask for a half a loaf of bread, you are going to end up with crumbs. If you ask for a full loaf of bread, you might end up with something." The biggest attack on Bernie is that his ideas are too ambitious ... they aren't realistic. His comments lead me to believe that he knows that, but he wants to begin negotiations with everything on the table, instead of conceding ground from the beginning. The disagreements he has with Hillary are not insurmountable. They have the same goals in mind, it is a matter of where the negotiations with those on the right and center right begin.
His biggest attack on her is about campaign finances, and that she takes money from the big financial institutions ... presumably for something in return. Republicans other than, perhaps, Trump will be able to make similar attacks. But Trump has so many other flaws, and has said so many outrageous, offensive and racist things that can be exploited. And the more outrageous and bigoted he gets, the more republican voters seem to fall in line with him. Some of it is accepting the inevitable on their part, I suspect. But there are enough Republicans who will not support a Trump presidency that he will not be able to unite the party fully to his cause (whatever the hell that might be).
In order to take on the Trump phenomenon, Hillary needs to solidify the support of all left-leaning voters. A pretty big chunk of those voters are in the Bernie camp, and if she is going to win, she needs them. For his part, I suspect Bernie understands this dynamic, and he is as put off by the prospect of a Trump presidency as anyone in the country. If he is the key to a Hillary victory, or more importantly a Trump defeat, I believe he will fall on his sword when the time comes. But even if he doesn't, Hillary needs to make overtures toward him to bring him into the fold. His support is heavily with young people -- many of them who have little experience with the political system. If their guy isn't on the ticket, many of them will bail on the election. If that happens, Trumps chances of winning rise. Bernie does not want that to happen any more than anybody.
Conventional wisdom prior to the Trump ascension was that she would choose one of the Castro brothers, to solidify the vital hispanic vote. Trump's racist rhetoric has made that a moot point. Given her widespread unpopularity (whether rational and realistic or not) he needs energy and enthusiasm in her campaign, because Trump's army of mouth breathers will come out in droves to the polls. If Hillary inserts another establishment stooge as her running mate, she will lose the election and we will be faced with a Trump presidency. I mean, the Republican voter turnout for the primaries has been off the charts. We can convince ourselves that that is because so many anti-Trump voters are showing up to keep him from office, but we would be lying to ourselves. In my view, Bernie is the only choice that will ensure the party has a unified party to face Trump in the general.
It will be tough, certainly, to overcome the jabs Hillary and Bernie have thrown at each other in the primaries. And it will be tough for Bernie to moderate his strong beliefs to support a centrist candidate, but I believe he can manage that pretty easy. I mean, they will be running against a guy who has Republicans in open rebellion at the prospect of his candidacy. Each new day brings a handful of new Republican leaders who say they will not support him in the general. His primary opponents have called him a con man, a racist, a danger to the country with a tiny penis. Any disagreements that Hillary and Bernie have had will be small potatoes compared to the bombs that are and will continue to be lobbed at Trump from his own side.
The Clintons are all about winning ... and Bernie as VP gives them the best chance to win. The at least need to put the ball in Bernie's court. I'm not saying he'll pick it up and run with it, but the overture will help some, I suspect. And if he accepts the VP nod, I feel a whole lot better about keeping Trump from the Oval Office. I think there is little doubt that this will be a topic of discussion as the fate of Bernie becomes clearer and we head toward the Democratic Convention. Anyway, that's how I see it.