I would agree, except for a couple points. First, to the arguement that since there are only two Sith, then the old master would have to be done away with. This does not account for Vader and the Emperor’s discussion in the Holotheatre in TESB. “If he could be turned, he would be a powerful ally.” “Yes…yes. Can it be done?” “He will join us or die.”
This brings up some interesting points. First, it is unclear whether Vader knew that only 2 Sith can exist at one time. If he didn’t, (which I believe is the case, due to the ‘us’ in his wording) then he wouldn’t have realized that by offering to turn Luke he would be signing either his own or the Emperor’s death warrant. This would show that when Vader proposed Luke to join him on Cloud City, he was doing so without having the ‘natural way of things’, but more of an overthrow of power. (This ties in later)
However, if he did know that only 2 Sith can exist at a time, then Vader knew that there would be 1 of 3 different outcomes when he offered to turn Luke. The first would be that he was successful, and the Luke would help him overthrow the Emperor. The second would be that the Emperor would kill Vader and take Luke as his apprentice. This would be the most likely scenario, as Vader was less powerful than the Emperor, and Luke, while he CAN destroy the Emperor, has not focused his power. (Pointed out by Vader during their duel on Cloud City) The Emperor would simply be able to kill Vader and take Luke as his apprentice. And then there’s scenario three, where Vader (or the Emperor) kills Luke, and life goes on as normal.
Now, when the Emperor agreed to turning Luke, (this is still assuming Vader knew about only 2 Sith at one time) Vader realized that the Emperor was planning his death already. Since Luke was obviously more powerful, then Vader would be the odd-man out, and the Emperor would kill him. At that point, Vader’s devotion to the the Emperor would have been destroyed. Since he is a Dark Jedi, his lust for power would simply overrule that. We also see his need for power in the other movies as well… Choking Admiral Motti on the Death Star, was simply an act to show that his power, the Force, was much more than anything the lowly Admiral could create. When Vader feels his power is threatened, he gets mad and retaliates. This is also shown in his duel on Cloud City. Luke’s unwillingness to turn was a direct challenge to his skill, so he got mad. Best shown just after Luke is thrown out the window and is trying to find his way back in. If you watch, Vader’s fighting is much more ‘enraged’ looking.
So in conclusion, I’ve just rambled on, and probably lost many of you. But my point is, Vader’s motives were not primarily due to his wanting to save his son, and they definitely weren’t out of loyalty to his Emperor. I could probably go into more detail, but I’ve got pizza and homework to do, so I’ll check back in later.