i think vader's plan was to allow luke to kill the emporer, based on a misunderstanding...an intentional 'sin of omission' on the emporer's part.
i believe it was impossible for luke to do it...not like everyone planned anyway.
and as far as vader's ambitions...he wasnt quite up to speed with palps'
plans from what i've seen in the OT. one simple line tells me that
"he will come to me?". palps is the giver of plans, visions, and information...
where vader is on a 'need to know' basis.
i believe that palps suspected that vader's son could bring anakin
back to the good side and in turn destroy the sith by reminding anakin
of his true identity and his destiny. i believe he kept the details away from
vader and simply said "he could destroy us", to appeal to vader's love for power. it would make sense for him to do so, as vader is still evil, and wants to keep his power.
so vader secretly offers up the possability of sparing luke AND taking absolute power in the sith, based on his misunderstanding of palp's vision.
"you can destroy the emporer! he has forseen this"
well...as far as i can tell palps visions are quite clear right down to the last detail. how could he not suspect the return of anakin? of coarse he did.
"i wonder if your feelings on this matter are clear lord vader..."
so yeah, i think palps forsaw the end of RotJ and manipulated vader in an attempt to not only avoid his fate, but gain a new apprentice...AND get rid
of his untrustworthy half-a-man apprentice.