What is it that Obi-Wan and Yoda want Luke to do exactly?

It is vaguely expressed that Luke must confront Darth Vader in order to become a Jedi. It appears to be clear that Luke believes, counter to their wisdom, that he can bring out the good in his father. And when on the Death Star, the Emperor clearly tries to seduce Luke to anger by encouraging him to either kill him or Vader.

What should Luke have done according to Obi-Wan and Yoda? They clearly don’t believe in being able to save Vader from the dark side, and I have to believe they want him to kill Vader. But this appears to also be what the Emperor wants.

Surely if Vader was not believed to have any good in him left, this could not have been what Obi-Wan and Yoda wanted their student to gamble on.