The idea that Revan or Scourge grew more in power between the novel and the game than Vitiate did is beyond laughable. This is somebody who has more innate potential and has access to far more dark side knowledge reserves and rituals/experiments. Scourge spent his time fighting no-names and Revan spent his three centuries in a prison fighting a mental war and not doing anything physically.
Revan learnt directly from Vitiate's mind for three hundred years(presumably also the Dread Master's), became more powerful by the Foundry and then is stated to be more intensely powerful as Dark Revan, so much stronger he goes from losing to Act 2 Imperial protagonists to giving extremely difficult fights to two strike teams before being defeated fully.
We can assume Vitiate grew, but really he was splitting his attention and power across multiple fronts by SWTOR and Ziost is the only ldgitimate spurce we have, but I assume this is not Ziost Vitiate.
Revan with full rest and restored health, followed by a massive increase in knowledge and two apparent power increases, is clearly much more powerful.
Valkorion would win this but not SWTOR Vitiate.