General Kaliero
F = ma, beeyotches.
Vader in ESB is a man who has realized that Palpatine sees the value of his son, and intends to replace him (Vader), with a newer model. When the two speak, Vader says "He will join us or die."
Now, think about this. A mere brutal monster would simply kill the younger rival for his position, saying he had remained true to his word and nothing could be done, keeping his role in relative safety with the last of the Jedi dead.
Does Vader do that? No, he executes a plan that involves turning his son, offering the chance to rule together. Remember, Vader fells to the Dark Side not out of rage, but out of desperate compassion for his wife. Listen to him when he speaks to Luke in Cloud City.
There is no escape. Don't make me destroy you. You do not yet realize your importance. You have only begun to discover you power. Join me and I will complete your training. With our combined strength, we can end this destructive conflict and bring order to the galaxy.
Vader does not want to kill his son, who is his only relation that he knows to be alive at this point. He wants the companionship that he had lost with the death of Padme.
Join me, and we can rule the galaxy as father and son.
Vader has only just revealed their connection to Luke a moment before. Luke reacted in anguish - not exactly the best indicator that the kid's happy with this news. And yet Vader
insists on reaffirming their connection. This is not for the purpose of convincing Luke; it is Vader practically
begging someone he wants to care for to simply be with him instead of against him.
After Luke escapes on the Falcon, Vader orders his troops to set their weapons to stun as they capture the ship, again not even thinking of how easy it would be to explain Luke's accidental death during recapture - an out that he clearly gave himself when speaking with the Emperor, were he personally willing to take it.
He reaches out to Luke again through the Force, once again calling him "Son," and only quietly asks that he join him. And once the Falcon has escaped for good? Vader can't even react with his typical rage. He lets Piett live, and slowly walks off the bridge in a manner the official script terms "contemplative."
Vader in ESB is a trapped father first, a villain second. Nearly everything he does concerning Luke points to that fact.