I think you can have powerful willpower and not per-se be a powerful telepathy, but I don't think it's possible you can have good tp and not have good willpower, too.
Sources such Dark Journey directly correlated Kyp Durron's telepathic ability with that of his willpower.
As for the fight, Vader obvs loses.
Each Dread Master is a master of the Dark Side of the Force with a great deal of specialization in Sith sorcery.
They are also proficient in team-work, to the point that they have decimated entire fleets out in the space.
This contest is a mismatch in the favor of the Dread Masters collective. As good as Darth Vader is in single combat, no chance for him to succeed here.