I know that Lucas said that Sidious is the most powerful sith lord ever bla bla bla...
You have to realize that the Star Wars universe is constantly expanding - some stuff will get retconned to lesser or greater extent. It is inevitable.
Just read Revan novel and play JK storyline and ask yourself who is superior - Vitiate or Sidious.
Here is some nice quote from codex entry related to the Emperor:
After you find out more about the Emperor you realize what kind of creature of pure evil he is. The way he speaks, everything about him is just terrifying and causes chills to go down your spine. I mean, he killed a sith lord when he was like what, twelve years old? When he was an infant, his eyes were filled with terrible blackness and void (Wth?! How sick and creepy is that?).
He is unlike any being that was ever born, when Lord Scourge who was a Sith Lord took a not even second long glimpse at the Emperor, he cried out like a child.
Some of his abilities on top of my head:
- Being able to perform most complex sith rituals ever attempted (consuming life of the entire planet - billions of live beings destroyed in matters of seconds).
- Being immortal? "His life spans millenia". Also from what I understand, it is almost impossible to completely kill the Emperor. His essence will always live on, attempting to find another host it can possess. For sure the Emperor got some weakness that can be exploited in order to defeat him, as everyone, but noone knows what it is yet.
- Being able to mind control pretty much anyone (though it is possible to resist the Emperor, Revan being the proof of that.
I am pretty sure that it is BioWare's intention to make us feel that Vitiate is in fact the most powerful, evil and dangerous sith/living being to ever exist (including the movies and all post EP6 expanded universe).