Vader, for all his power, is still human and still has compassion (much as he tries to deny it). Sauron is a borderline satanic entity with no scruples or humanity. There's no way Vader could take him.
That being said, it would be a tremendous fight to watch. Vader would go down, but he'd make it as painful and difficult for Sauron as possible.
Wasn't it a human who defeated Sauron in the first place? It was only after Sauron cut through about a hundred men, but a human still defeated him. You have to keep in mind Vader is technologically superior, what with his lightsabre. I'm not saying Vader would win indefinitely, but one wrong move from either of them, and the other could end it.