Like with Obi-Wan it's not quite clear if this is Vader, Anakin, or both in flashbacks or present. Given that the show probably takes place mostly during the time of the Mandalorian, after Vader's dead, I'm guessing these might be flashbacks (With slightly De-aged Hayden? He hasn't aged really that badly; if anything he seems to have mostly loss some of the muscle he bulked up to play Anakin in the first place-and of course the hair is easy). Or perhaps Anakin's force ghost, which appeared at the end of ROTJ of course (With of course the Hayden likeness added in 2004's DVD) and was heard in ROS. Maybe they can finally do that Anakin force ghost which transitions to Vader and back again that was in several concept art pieces.
It might be interesting to see a flashback of Anakin perhaps wearing the armor with the somewhat Vader-esque shoulders from the early seasons (and 'borrowed' from the Gendy series).