Possible Spoilers ahead.
Rogue One takes place just before A New Hope putting it approximately 19 years after the events of Revenge of the Sith. Seeing how it's been nearly two decades since his dismemberment and burning, what's the point of putting Vader in a bacta bath now? If it would have really helped him regenerate, Palpatine would have had him in one soon after his defeat on Mustafar, but instead immediately took him to surgery and had cybernetics attached. I also don't think it had any pain management purpose because Dark Side users often draw on pain to fuel their Force abilities, so it is unlikely a Sith Lord would try to reduce their own pain.