But hey, it never actually happened did it so its not such a big danger after all. The reason why the Rule failed was because Vader abandoned it, really Sidious should have realised his apprentice was weak and broken and destroyed him - then found a new one. That is what Bane would have done.
I dont think vader was any of those things to be honest even in his suit. Weak because he turn to the light side in the end? i see it the oposite only make it even more stronger. I can not imagine Palpatine ever doing the same, him being so consumed by evil and the darkside. But Vader was the chosen one though a blatant exception.
The whole sith ideology is retarded though and doomed to fail. A ideology that promotes darwinism, and selfisness, its very interesting but not very logical and prone to backfire sooner or later. A thing that constantly destroys seeking to build something into perfection is a contradictory ilusion. They can gain power, but they are doomed to lose it someday. The problem is the damage that causes in the meanwhile.