Questions about 1st-movie-only canon and ep4+5-only canon

[Havok]

He says "Offspring of Anakin Skywalker" (that is the updated version) to which Vader responds, "How is that possible?"
It is in the Emperor hologram scene.

The original runs...
"What is thy bidding, my master."
"There is a great disturbance in the Force."
"I have felt it."
"We have a knew enemy, Luke Skywalker."
"Yes my master."
"He could destroy us."
"He is just a boy. Obi-Wan can no longer help."
"The Force is strong with him. The son of Skywalker must not become a Jedi."
"If he could be turned, he would become a powerful ally."
"Yes. Yesss. He would be a great asset. Can it be done?"
"He will join us or die, master."

The new version...
"What is thy bidding, my master."
"There is a great disturbance in the Force."
"I have felt it."
"We have a knew enemy. The young rebel who destroyed the Death Star. I have no doubt that this boy is the offspring of Anakin Skywalker."
"How is that possible?"
"Search your feelings Lord Vader. You will know it to be true. He could destroy us."
"He is just a boy. Obi-Wan can no longer help."
"The Force is strong with him. The son of Skywalker must not become a Jedi."
"If he could be turned, he would become a powerful ally."
"Yes. Yesss. He would be a great asset. Can it be done?"
"He will join us or die, master."

In the original, it is clear that Vader is hunting for his son and he knows it, even though, the Emperor just found out.
In the new version, it is clear, it seems like he may have just been hunting the people that blew up the Death Star and the rebels in general, and Palpatine sets him on the path towards Luke.

[Tiriol]

I actually got the implication that both Sith Lords were toying with each other in that scene. Listen to their tones: both are monotone, almost emotionless. Vader particularly stands out. It's like two liars who both know that they are lying to each other but doing it anyway.

The fact that the Emperor decided to speak about offspring of Skywalker to Vader could show that Vader had no idea about Luke's actual identity (pretty doubtful, though, considering that the opening crawl still states that "the evil Lord Darth Vader, obsessed with young Skywalker", which means that he's the biggest idiot in the galaxy if he had no suspicions about the similar surname); Palpatine genuinely wanted to inform his apprentice about this newest development since he also genuinely believed that Vader didn't know about it (Palpatine is no idiot, either, though, and used to treachery, so this seems unlikely); or that Palpatine wanted to make a veiled reminder to Vader who is still in charge and who still knows what goes on in the galaxy.

Personally I support the third option.

[Lord Insanity]

Keep in mind in both the original and updated ESB, Vader (upon seeing the Hoth base image from the probe droid) states: "That's it, the Rebels are there and I'm sure Skywalker is with them." This was before his conversation with the Emperor. Its obvious the implication is he knew who Luke was before the conversation with Palpatine and was simply "playing dumb" during it.

[Darth Tedious]

Regardless of all that, it isn't until RotJ that Obi-Wan explains that Darth Vader and Anakin Skywalker are the same person.

If you only take episodes 4 and 5 as canon, it remains entirely possible that Vader was just fucking with Luke's head at Bespin. Personally, I love the idea that it was all a big psyche-out.

Edit: Or, even better, Vader was Luke's father, but is still a seperate person to Anakin, giving him the "Haha, I did your mom!!!1!" factor! Ultimate psyche-out!

[Stark]

Keep in mind in both the original and updated ESB, Vader (upon seeing the Hoth base image from the probe droid) states: "That's it, the Rebels are there and I'm sure Skywalker is with them." This was before his conversation with the Emperor. Its obvious the implication is he knew who Luke was before the conversation with Palpatine and was simply "playing dumb" during it.

A possibly more interesting idea is that he has suffered a complete psychic break, and no longer considers himself 'Anakin Skywalker'. In this case, the ESB revelation becomes his first step on the road to redemption, simply by admitting he is himself.

[Galvatron]

- please tell me more. Anything and everything you can about this.

Galvatron is a good guy to talk to about this. Drop him a PM if you want to discuss it some more.

Or he could just read this old thread.