Given the fact that the emperor doesn't even grace his appearance to anyone, it'd just be "The Voice of the Emperor" that would kick everyone's arse. lol.
Hmm, ok let's say in this hypothetical bout that the Sith Emperor deals with the problem in person. Let's say it's in the core of the Korriban Academy, and there's no escape routes for any of the battlers. With their combined Force connections and lightsaber skills, I think the 5 Sith Lords could put up a fight. Don't get me wrong, I still think Darth Vitiate would annihilate them. But, I think the combined knowledge of the Sith Lords could see them hold their own against the Emperor for a while.
I can't see Darth Nihilus or Darth Traya being indoctrinated by the Emperor like Revan and Malak. Also, I'm sure together they could heighten their Force powers to resist Darth Vitiate long enough to give him some problems.