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* Vader says, "You underestimate the power of the Dark Side--I MUST obey my master." Why then did he offer to "Rule the galaxy as father and son" in TESB?
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This is a very roughshod explanation, but from reading various books I've gathered that the Sith is always just two people - a master and a student/servant. Eventually the student will try to destroy his master, take his place, and choose his own student.
Unfortunately for Vader, he was almost destroyed himself before being saved by Palpatine. As a result, he never would have had enough power to overthrow the Emperor by himself. However, with the help of a freshly turned Luke, they could destroy him together.
Whether or not Lucas had this in mind when he wrote this story is up to debate (and like a lot of things in the movies/universe probably wasn't).
EDIT:
http://en.wikipedia.org/wiki/Sith - This mentions the "always two" rule among other things.