Sidious is the only one I think could present a reasonable challenge to Vitiate. Even then I don't think it would be particularly even. He was absurdly powerful even before SWTOR elevated him to frankly silly levels.

@Snowskeeper What about the Son?

@Grimlord31

I'll admit I don't know a lot about him, since I haven't watched the Clone Wars series, but based on his wiki page I'm not seeing a lot that convinces me he could take Vitiate. Vitiate ate entire planets, hopped between bodies like a frog between lily pads, and came back from 'full' death like four times; the Son is very powerful in the Dark Side, but his main powers seem to revolve around corruption and manipulation, which Vitiate is extremely not vulnerable to. The Son's immortality is more of a problem, but given Vitiate spent close to a millennia studying the Dark Side in seclusion, and has a lot of experience suppressing or removing the Force abilities of others, I would expect him to be able to work out how to deal with that. I don't think the Son has anything that would allow him to permanently kill Vitiate, unless I'm missing something.

Wanna be super clear again, though, that I haven't watched Clone Wars, so I may be missing some important details about him that would change how this matchup works. If I am, I apologize.