Vader holds off Plagueis while Ulic and Dooku stomp Nihilus
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk
I'd say this depends on how well Nihilus performs. If his drain can perform, or if you can substantiate his TK as being godly, his team could win.
Vader would contend with Plagueis for a time, but he has no chance of victory, and it wouldn't be a terribly long fight. Along with the sizeable power difference, Plagueis is way too fast and agile for Vader to contend with in a duel seriously. Things only get worse for Ulic and Dooku. Plagueis versus all 3 would be a pretty good fight, but I'm not convinced he'd win...
__________________ “The galaxy must experience the pain of death and the rapture of rebirth as I have. I will bring chaos. It is time for war.”
Vader scaling over Mace per his scaling over Kar Vastor (both were pre-prime when they were compared to him) gives him an avenue of scaling that gives him some favorable comparison to Sidious, While context makes clear that Mace beating/stalemating Sidious is solely down to circumstances, there's nothing that stops sidious from being able to compete with Sidious by virtue of his own merit.
In fact, factually, per word of god, Mace can compete in general. The context of the quote does not limit this to be exclusively saber combat(and mace can compete in a duel by virtue of being an eight bordering nine). Thus we can conclude Sidious can't one-shot him with the force, which would render him being able to compete with Sidious bunk, which would allow us to dismiss Sidious being able to outright dominate him.
Additionally, Mace, much like Jedi Anakin, has hype that puts him in Yoda's general range with there even being quotes suggesting Mace's superiority/parity(though that's obviously not the case via thevast amount of quotes that say otherwise).
This means Vader gets some sort of favorable comparison to Sidious which is more than I ca say for Plagueis. All we know about Plagueis regarding Sidious is that he was substantially below a pre-prime Sids.
That being said, Plagueis has a key advantage in terms of hax and variety. Especially regarding mediclhorian manipulation which was able to counter tpm sid's lightning. Such a tool could likely negate any tk edge Vader may or may not have.
What's unclear to me is how the two compare as duelists which could decide this match.
****ing scaling is such a boring argument. Vader has no decent showings apart from beating fodder tier Jedi and getting outperformed by a sub Ventress duelist in Marek. Only a couple of impressive force feats, none of which put him on Plagueis' level