With the Valley of the Jedi as a source then yes, I can't see Vitiate winning easily at all, if he wins it's a crazy hard fight. With Nomi's BM to block sorcery, Thon with his WoL and Jeth with his own, that's going to be seriously strong.
Potentially they could combine their amped powers to WoL him, at any rate this is a seriously good team.
From his showings in the novel the team could potentially win; however with the new information brought forward by KotFE stating that Vitiate never showed his full potential in any battle ever conducted as Vitiate (his only true showings being as Valkorion), I'm thinking that Vitiate will win this. Assuming he had most of his focus on Valkorion rather than Vitiate during his Revan fight, he definitely wins this. However, assuming this is only using his feats seen up to and in the novel, discounting the factor of the majority of his presence being elsewhere, he loses in a good fight.
Edit: assuming only Vitiate body feats, wins 1 in VERY close fight but loses 2 in another good one. Assuming his full focus is on Vitiate (total disconnection from Valkorion) he wins both solidly.