More important, by the time Vader was capable of becoming a risk to his Mastery, Sidious would be fully conversant with the secrets Plagueis had spent a lifetime seeking–the power of life over death. There would be no need to fear Vader.
-- Dark Lord: The Rise Of Darth Vader
Yes, Vader remained strong in the Force, but strong enough to succeed me? Never. Granted, he was strong enough to kill me. But that only lasted for so long.
-- Jedi vs. Sith: The Essential Guide To The Force
Somewhere in the back of Palpatine's mind, he knows what Vader's potential is. He feels he may be wrong, but he is wary of it. So he's very interested in determining his ability to manipulate Vader and testing his loyalty and assuring himself that this tiger that he holds by the tail is going to stay that way.
-- Insider 157
FFS is that hard to hit up a ****ing respect thread? 90% of these threads wouldn't exist otherwise.
Hint: This is how you do it:
http://lmgtfy.com/?q=darth+vader+respect+thread#