Vader was voiced in Rebels, afaik by James Earl Jones, that is not possible now and I think I read somewhere that he gives permission to Lucasarts to recreate his voice, using ai or whatever, my guess is that permission will cost something to Disney.
It's all guessing on my part, but Vader is there because Palpatine sent him to kill Maul. I think there is something to say. Lets see if there is some kind of interviews or something and someone say something about it