Vader wins with just two words: "Imperial Interdiction."
Soth can do whatever he wants on the planet, but he can never leave - say to find and confront Vader again. Vader in contrast can go wherever in the galaxy he wants and do whatever needs doing, untroubled by Soth.
P.S. Absent some stats to compare, the question has no true solution.
Soth and Vader exchanges strikes inconclusively. The magic of Soth's armor is able to block a lightsaber and Vader's full suite is impervious to the feeble warriors strikes. Mastery in magic and the Force as similarly proven equally useless as each other can cancel the other's attack and they resorted to a sword fight.
Soth: "I'll kill you and play with your innards, using your intestine as a lasso to strangle your followers!"
Vader... ponders for a second.
Soth: "You'll soon die, male inheritor of a she-dog!" [the fight is narrated in a book for family audience]
Vader: "Soth... I am your father."
Vader (chopping his hand off): "it works e-ve-ry single time."