When Vader says "Give yourself to the Darkside. It is the only way you can save your friends" I've always wondered what he actually means. Luke's friends are either on Endor or battling the Death Star as part of the Rebel Fleet and engaged in battle with the Imperials at the time. The Emperor has already used this in an attempt to stir anger in Luke. Could it be because Vader himself was promised the ability to save the person he loved when he turned to the Darkside by Palpatine? Or is he saying that to provoke Luke into thinking of his love ones so Vader could try and probe his mind for something to use as leverage? Personally I think it's the latter, as it did make Luke think of his sister - the person he most feared to lose.