I don't really see what the huge disadvantage of a split vote would be for Bernie supporters. Let's take it as granted that they can't ever support Trump (given that Trump and Bernie share nationalism, commitment to the American worker, distaste for open borders among other things I don't think that it would be impossible for a Bernie voter to support Trump, but let's say that it is for the sake of argument).
Even with Trump as unacceptable, its not clear why Hillary should be acceptable. After all, she has greatly benefited from many of the very types of corruption and abuse of power that Bernie railed against and that his supporters hate. What's more, she used the same sort of corrupt means to do everything in her power to minimize Bernie and shut out his supporters. What are the chances that she's going to change her mind and start wooing Bernie's voters now? If she had any intention of doing that she would have done it in the primaries, instead of rigging the election, but at this point she has no reason to since they are assumed to be in the bag for her.
So it looks like an America lead by Hillary or Trump would be a disaster for a Bernie voter. Okay, but if that is the case, there isn't a strong preference for either. So to determine how to vote, they should look at other factors. And the most relevant would seem to be: how to prevent the primaries from being rigged again? The answer is to make sure that the DNC and the Democrats suffer for the rigging that they performed this year. A good way to do that would be to vote for a third party candidate who is closer to their views anyway. That would split the vote and get Trump in office, but so what? The alternative was Hillary. However, doing this they have a strong bargaining chip in the next round of primaries: they can tell the DNC that if they rig things against the will of the people again, then they will lose another presidential election.