And "up to the viewer" is perfect. Everybody gets whatever version of Vader they think works best at the end of ESB.
For me, it's not even so much a "softening" of Vader at that point as it is simply the *beginning* of an inner conflict. In my mind, Vader didn't do anything nearly as dramatic as flipping a switch and start abandoning his connection with the dark side. But I think he was confronting feelings he wouldn't have been conflicted by for two decades.
I know that this is going to seem like I'm making way too much out of it, but I'll say it anyway. When Luke chose to fall (perhaps to his death) rather than join the dark side, Vader/Anakin was literally watching his son going to a complete extreme to make the opposite choice from what Anakin did 20 years earlier. I imagine somewhere in Vader's head he was thinking, "my kid is stronger/better than I was."
Whatever Vader thought of Luke before their Bespin duel *had* to change by the end of the film, IMO. And I think it made him reflect introspectively when comparing himself to his son. Seeing his own flesh and blood choose another path wouldn't "soften" Vader, but I think it would cause him to recognize what might've been had his own choices gone a different way. He could've raised that son who had just earned his respect (presumably).
Vader was sincere in ROTJ when he told Luke, "it's too late for me, son" but he knew that it wasn't too late for Luke. So for me, that last scene in ESB is Vader questioning how hard he should try to bring Luke into the dark side. That bit of conflict is a new wrinkle in the Vader persona. Just my take, of course.