I'm assuming speed is equalised.
Firstly, I'm sure that Vader has the AP advantage, since you could create a laughably long scaling chain from Yarael Poof who's planet level feat is already a bit beyond baseline iirc.
It doesn't matter since SW might be getting downgraded to High 6-A
The make or break thing here is hax. I have no clue as to if Ollerus is some sort of tier 1 smurf or not. That power null seems really problematic, but based on
this thread it apparently can only be used on "Gods"?
The other big thing in Ollerus' favour is the hundreds of millions of explosions that apparently spawn on every part of the target's body.
I do not know of the level of mind hax resistance Ollerus possesses. If someone could tell me that would be great
As for Vader, while I am unsure of the specifics of his mind hax potential in Legends, I'm at least fairly certain its a fair bit beyond baseline. Some input from Vader experts would be pretty cool here.
Vader's precognition lets him predict his opponent's actions meaning he can be alerted to Ollerus'
intent to perform the hundred million spam attack and it would be quite in character for him to just rush Ollerus and cut him up in that case. High-Low regeneration isn't enough to come back from something as severe as decapitation for example.
Anyway, because I don't know exactly the potency of Ollerus' hax and resistances I can't make a judgement as to who would win.