It's simply a case of Tenny having more going for him, Banite scaling combined with his proximity to Plagueis leaves Vader struggling to keep up. And I don't really think Vader compares to the late Banite Sith at all, nor do meaningless stats from Lucas make for a compelling argument in that regard.
Originally posted by cs_zoltan'Cause Starkiller made Sheev horny?
Shitting all over Starkiller puts Vader close to Plagueis. Tenebrous joins Bane in the grave.
Whilst Lightning and speed/agility are pretty badly overblown as Vader's weaknesses, Tenebrous does have those advantage nevertheless, and with respects to general ability, it isn't hard to see Tenebrous being on Vader's level at all, if not better.
Personally, I see Vader being closer to Bane than to Plagueis, so Tenebrous is indeed superior.