C3PX said:
I dunno, even as a guy on fire who is slowly sliding into a molten river, he fails as Darth Vader. The Darth Vader I know would have used the force to force choke the piss out of Obi-Wan who he standing on the bank expressing his disappointment and undying love for Anakin while simultaniously watching his flesh burn off of his body.

Or loosened the gravel under Obi-Wan's feet, so he lost his footing. Or hurled lava over his own body towards Obi-Wan. Mustafar had too interesting of a terrain for my version of Vader not to exploit it.

That's why I think it would have been more acceptable to have Vader become immediately "unavailable" after being de-limbed. No romantic confessions, no exchange of words at all. As soon as he's defeated, Vader is somehow dragged away from the scene.

I've spent accumulated hours thinking of a scenario of how to do this. My favorite one I've come up with is Obi-Wan Force Pushing Vader (after he was de-limbed), off a giant narrow cliff. Obi-Wan pries Vader's lightsaber from the hand on the disembodied arm, he walks to the edge of the cliff, looks down far below, but doesn't see Vader. So he assumes Vader fell straight into the river and was destroyed.

Then we don't see Vader until ANH. We don't need to know exactly where he landed. All we need to know is that Obi-Wan defeated him, and he was horrifically injured further from the lava afterwards (a given because of the new suit, iron lung, and burn marks later revealed in ROTJ).