There has been much debate, but here is the truth. Vader is sith by name only. He doesn't even possess the guts to be a true sith (not to mention any guts at all...). True Sith are, or were a red-skinned race native of Korriban, until fallen Jedi came and made themselves gods. Then imbreeding started, and Jedi and Sith merged to create what all good Jedi fear, the Dark Lords of the Sith. Some of the originals consist of Tulak Hord, Naga Sadow, Ludo Kresh, Ajunta Pall. They were ruthless, cold-hearted killers. And Power-Hungry. The rules of the Sith were simple. The stronger were on top. Apprentices killed their masters when the masters became too weak. But they were so power-hungry that they wiped themselves out. Then the posers came. Exar Kun, Ulic Qel-Droma, Darth Revan and Malak who started the "Darth" trend. Later on came Darth Bane, who implanted the Two-Sith Rule. Then they became weaker and weaker. Vader the most pathetic. If he were anywhere near sith, he would've slaughtered luke in Bespin. And then Palpatine. He could've waltzed right into the rebel bases and wiped them all out. All in all, him calling himself a Sith is unjustified to the real sith, who would've slaughted him into electric sushi the moment he left bespin.