I have a feeling that the lava planet was added later and the original idea was that Vader was on a Stardestroyer. I don’t have much to base that on, so I could very well be wrong.
It would have been nice if he was on his big Destroyer from ESB. I know some would complain that it would ruin that ships reveal in ESB, but I see this film as a bonus story to the OT, not something to watch before it.
Well it would certainly be nice to see executor in R1. However, I have always assumed that it was constructed sometime between ANH and ESB, since Vader’s flagship was a normal ISD in ANH.
Still I would personally prefer if they met on a ship (even if just ISD) instead of lava planet. It would give the feeling that Vader is constantly doing something instead of sitting somewhere.