Vitiate is definitely more powerful imo, but Plagueis is just a more skilled combatant; he has better speed feats and his proficiency with a lightsaber is very underrated, and the gap in force power isn't enough to earn Vitiate the win with sorcery here.
I have gained a new respect for Vitiate though after reading more and more material on him. I'd have him as my 4th "best" Sith after Caedus and Plagueis.
I'd have to say Vitiate wins this. Vitiate delved into the deepest secrets of the Dark Side and used the darkest rituals, where Pagueis just learned to manipulate the Force and make it do what he wanted it to, rather than learn to kill things and such. He definitely has better saber feats, though I'm not sure it would become a melee battle. Plus we've never seen Vitiates full potential, other than when he wiped out the Jedi strike team. I think that he probably has some crazy Dark Side power or ritual or something that he can use to cripple Plagueis. There are accounts stating he one-shotted DC members, and we don't know what he used or did for that, so I'm thinking Viti can handle this no problem.
^^^
Boast all you want but Plagueis is relatively much less tested in combat situations then the Emperor.
Emperor have defeated some of the greatest warriors of the Jedi Order with laughable ease, even felled entire Strike Teams of powerful Jedi and Sith with his powers. Emperor have superior feats and showings holistically.
Vitiate has stronger abilities in the esoteric areas of the Dark Side by quite a degree.
However Plagueis has displayed greater TK in combat, he has magnitudes greater speed, far more durability from fatal wounds as well as excellent Force defense. A lightsaber proficiency comparison would simply be a joke.
Plagueis wins as long as he keeps it short and close.