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No backstabbing? Making that offer to Luke far away from the Emperor, with no witnesses around.... that's backstabbing already.
I think you're missing the point JYAMG, I see that line as just a tempting false offer. Just because he said it doesn't mean he meant it. He would say anything to get Luke to follow him.
As for Vader not being an idiot, well neither is the Emperor. What good is it to have a Sith apprentice if he is liable to backstabb? In that case the Emperor would be better off killing him instead.
The whole point with the Emperor turning Anakin (and later trying to turn Luke) to the dark side is because he can control them and make use of them. If they are still a threat, why bother. He should just kill them instead. If Vader had plans to betray his "master" then this whole "turn to the dark side" stuff would be higly illogical. One can argue that Vader did indeed betray his master, but the only reason why Vader turned to the light was because of his son.
And to have Luke turn to the dark side as well, kill the Emperor and battle with Vader would be cool yes, but it would hardly be "Return of the Jedi", more like "Another Sith is born...." and that would make the Emperor look even more foolish:
"Yes your journey towards the dark side is complete! Let the hate flow... wait... wait.. Luke what are you doing? Put that lightsaber.... AAAAAHHH"