I'd also add that thre way they interact in ANH indicates Kenobi has seen him in the suit before. Vader isn't doing any "Look what you turned me into" stuff and Kenobi is casual about it. Obviously a function of Lucas not having that part of the story mapped out at the time, but it leaves an opening for a meeting between Kenobi and a fully suited Vader.
Don't forget that he "lied" to Luke about Vader killing his father. He didn't want to tell him that he BECAME Vader so them having crossed paths since Mustafar isn't a plot hole at all.