Fwiw I loved the Darth vorhees scene. Does it fit with what happens at the start of sw77…nooooooooo! But it is Vader and it is kick ass. And does match up with the separatist slaughter on mustafar. And the sandpeople masacre.
Also He becomes Vader after Mace L Jackson gets killed by big papa palpy. Starts turning at the sandperson slaughter.

As for the conversation on viewing order I would suggest
SW77
ESB
Your favorite 3 in one PT
(Shows the fall of vader so you might care)
Rouge one (still iffy on this but it would be more difficult to see enraged vader after his redemption in rotj)
ROTJ
TFA