He made sneak attack. And yes, Vader could've beat Emperor during Ep III (or with little training from Palpy, maybe some 1-2 after Ep III he would've pwned without problems, if not during movie time). But after his ermm...injury, he had only some 80 % of Palpys strenght
Luke've beat Emperor easily later on and did many other such things too. He is the ultimate.
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Vader doesn't and didn't sneak. It's impossible to not hear his breathing, even in the prescence of force lightning.