The return of the jedi novelization does a better job of stating why vader does not disobey his master.
I think it was because the time was not right. That he believed under the emperor and his teaching Luke would flourish into a sith apprentice though not stated at the time. And then when Luke was mentally prepared he would help vader kill the emperor and vader would be the new emporor and luke his chief right hand man and supreme commander of the elite imperial forces. I don't think as written in 1983 Vader expected his master to betray him and have luke Kill him, he never fathomed the emperor knew of his treachery.
When now in the EU the very nature of the sith is treachery, and there are under the cult of bane only two Sith Lords at any given time.
I mean Dooku did not see it coming either when Sidious ordered Anakin to murder Dooku.Tyrannus in cold blood. He never intended Maul to be a permanent Apprentice as far as i'm aware. Too Much of an animal and beast who could never really rise to the greatness the true masters of the sith could achieve. He was more of a slave to his passions than in control of them. He was only a lackey for palpatine sidious when sidious wanted an equel or someone who could one day succeed him.
Who better than the jedis chosen, the one to bring balance to the force. Corrupt him and the Jedis hope is extinguished. The only thing the emperor and Vader never foresaw where padmes and anakin's twins. The last threat to palpatines power. Luke the only one able to re-ignite the jedi inside darth vader from a small spark.
Which is kind of laughable considering that palpatine knew Anakin was banging Padme every night, why would'nt there be children,lol.
And nobody else knew or suspected. Obi Wan was the only one who knew and kept it a secret, being like a father to Anakin since he was a small boy forgiving him his faults.
It is entirely ironic that out of Padme and Anakin's sin of forbidden love that the galaxies hope was born, proving the jedi's view of attachment as being wrong. Indeed it was Lukes attachment to Anakin that saved him when he was a sith lord named darth vader, attachment to the ideal that he was a good man inside. Or had the potential of goodness anyway.
In the novelization of revenge of the sith the new prophecy was that love would destroy the sith not directly said but that is what i got out of it, Vader finally in his last moments excepts and loves his son and with his last energy kills the emporor at the expense of his own life doing away with the sith. The prophecy was then achieved but not as the jedi thought it would be.
I always wondered did the good in luke and leia come from Padme? Or the love of the foster parents. I mean their is little good shown in Anakin in the prequels as a confused and horny emo teenager, so i guess Its the foster parents,lol.
I mean good and evil as an hereditary thing rather than a moral choice.