Played through the vampire side, got cured. But even having Serena Tag along for an attempted re-visit... She doesn't initiate the dialog about doing a half-soul trap. And having her make me a vampire doesn't help either, it continues to do the health drain thing.

Fixes?

I know if you are doing the dawnguard side you have to cure serana as well. But your on the vampire side, this is a very strange situation indeed. If you on pc you can use console commands to spawn in and out of soul cairn probably. Or you could use console commands to cure serana possibly. I haven't used pc since morrowind so I am not up to date on the limitations nowadays.

Xbox 360.

If you were a vampire when you went into there the first time, you have to be a vampire to go in there again, so try that, otherwise it's likely bugged.

Okay... All I needed to do was have Serena turn me back into a vamp... outside. If you turn back in the same room as the entrance, the game gets confused. But it's a fairly easy fix.

...I probably should have consulted the wiki first, like I usually do. Momentary lapse in judgement I guess.

Anyway, problem solved. Got several grand soul gems and a few black ones, so I went back and got easily cured.