Well, then just have Vader kill Anakin instead. If the Vader in the OT wasn't Anakin, imagine what would have happened in episode 6.

First, there's Obi-Wan and Luke's talk on Dagobah. Obi-Wan could go on to mention Vader's past and warn Luke of his anger towards Vader. Luke would have a conflict between his Jedi teachings and wanting to bring justice to Vader.

Then, there's the final duel. Luke would probably forgive Vader of his crimes and Vader could kill Palpatine. Either Vader dies, he survives and goes into exile, or he survives and helps Luke rebuild the Jedi. That or either Luke kills both Vader and Palpatine.