Given how the Obi-Wan series is not going to be as prolonged as Mandalorian, I don't see much opportunity to include Hayden as post-ROTS Vader, you could in theory show things taking place with the Empire and have scene of Vader unmasked through it, but otherwise, he's only be the guy in the suit and why bother bringing him back to take on essentially a stunt man's role.
I would wager his part will be more for flashback, probably a scene or set of scenes that show their friendship and tie to showing the pain Obi-Wan is living with over what happened.
I would wager his part will be more for flashback, probably a scene or set of scenes that show their friendship and tie to showing the pain Obi-Wan is living with over what happened.