"If he could be turned, he could be a powerful ally."
Why does Vader say this to the Emperor in ESB? Presumably the "only two" rule stands. The Emperor says in ROTJ, after all, "take your father's place at my side." And Vader tells Luke that he can complete his training and the two can defeat Palpatine.
If this is the case, then if Luke is turned, then either Vader or Palpy has to go. So Vader's more or less saying, "Hey, Emperor, you know what would be really cool? Why don't we have a contest to see who can turn Luke evil? Winner gets a new apprentice. Loser dies."