"...the only way you can save your friends"

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.

He means that if Luke turns to the dark side he'll be able to kill Palpatine and then make the rest of the Imperials surrender.

IMO Palps was trying to stir up the darkside in Luke by making claims like that because his feeling for them are strong and basically trying to put him into a trap...He does something similar to Anakin in ROTS when convincing him to join. He gave him the bait when he told him he could save Padme by making it seem like he knew how to do it...but if you notice when Anakin commits to the darkside Palps changes the story up by saying together they might possibly figure it out and thus was lying initially but by this time Anakin's feelings for Padme made him a slave to the darkside so it was too late for him. I guess Palps figures "hey if i suckered his father that way , it just might work on Luke too"

"Could it be because Vader himself was promised the ability to save the person he loved when he turned to the Darkside by Palpatine?"

yes. it's a nice tie in the prequel gave to the ot.

How would Vader possibly believe that after the dark side essentially led him to kill Padme, rather than save her? Unless you think he's lying to Luke, which I don't believe he is.

For me it's simple. Its basically "join the dark side and we will spare your friends - you will have a say in what happens.". Its a bargaining chip. He is offering Luke power. Yes, it certainly reflects the offer Sidious gave to Anakin to save Padme. But its through a different way. It was just another trick/manipulation by the Sith to get Luke to turn. It was something Sidious would do himself.

I'm sure Luke realised earlier that when the rebels looked doomed, he was being offered the chance to save them if he turned. I mean, if he joined Sidious then he would have the power to help them

But Luke learned a valuable lesson. He remembered his failure in the cave. He saw that if he turned he would just make matters worse for his friends. He saw that the dark side was really the path to oblivion. And he saw that he needed to let go of his attachments and fears else if he held on too tightly he would lose them anyway.

Its all part of his choice to throw down his saber and let go. He saw that he was becoming his father and that the Jedi thing was to not fight and accept whatever fate he faced.

I think that was the original intent, though it can certainly be looked at from a prequel perspective. Palpatine is at the center of the spider's web that is the Empire; he's got an entire legion of his best troops fighting on Endor, the Imperial Fleet above it, and the Death Star itself. With Palpatine out of the equation, that web unravels. The biggest threat, the Death Star, is certainly rendered useless, as the Imperial officers were waiting on Palpatine's command to fire the superlaser.

When I saw ROTS and watched ROTJ I actually saw it in this weird metaphoric reference light.

Anakin was provoked into killing Mace Windu because his one chance of learning immortality was threatened. He landed the blow to Mace and was responsible for his death which finally convinced him his fear and anger could make him more powerful. At the end of ROTS obviously Vader thinks he's powerful enough to overthrow the Emporer once he learns to save her, so he and padme can rule the galaxy together.

Now in ROTJ, Vader is trying to give Luke the same motivation to join as
he had gotten in Episode III. Luke has the fear of losing his friends and Leia, the same as Anakin with Padme. If Luke doesn't make a decision eventually, they will die. So Darth Vader and the Emporer kinda emphasize the whole 'if you come with us, you can save everyone.'point and try to manipulate Luke to strike at Vader or Palpataine with anger and fear. Once he does that he will feel the power and if he's anything like Vader he'll turn. By then Vader desperatly wants Luke to turn to side with him and rule the galaxy; ala him and Padme.

There are quite a lot of things in Star Wars that are a bit ambiguous.

I sometimes wish George Lucas himself would write a tell-all book, or a sort of Star Wars bible, answering common questions like these

But in a way, maybe it's more fun to not have definite answers
People get to make their own interpretations and make the stories more personal that way
We also wouldn't have as much to talk about on these forums

I've always thought about this too. What Vader is saying, I think, is if you turn to the Dark Side, it can have a direct impact on what is happening in the Galatic War per se. Did anyone ever notice when Luke turns to the dark side and runs Vader down at the end of their duel, the shield is no longer up?

It seems on one hand Palpatine is telling Luke that he has nothing to lose by joining the Darkside because everyone that he cares about is about to be lost (ie Han, Leia, Chewbacca, the entire Rebel Alliance etc) and that his turning to the Darkside is 'unavoidable'. Then Vader says turning to the Darkside will save his friends.

Poor Luke resorts to hiding from Vader in the darkness. He's so conflicted at this point that he doesn't know what to do. Eventually it's Vader's threat to put Leia through the same anguish that Luke was going through that strikes a nerve. The more I think about it the more it seems the Emperor and Vader were just trying to find the right angle to unleash Luke's hatred.