Hoping for a triumph over the Empire is completely different than hoping Darth Vader still has a shred of humanity left inside him.

[Even if it's technically still Anakin they're talking about, before all the armor and the heavy breathing. But Leia doesn't care about that technicality.]

Jedi are meant to destroy the Sith. And if that light is still in him, doesn't that just make things worse? Knowing he committed the atrocities he carried out while knowing full well they were the wrong thing to do.