At the end of Episode III we see:
DARTH VADER: (continuing) Where is Padme? Is she safe, is she all right?
DARTH SIDIOUS moves closer to the half droid/half man.
DARTH SIDIOUS: I'm afraid she died. ... it seems in your anger, you killed her.
A LOW GROAN emanates from Vader's mask. Suddenly everything in the room begins to implode, including some of the DROIDS.
DARTH VADER: I couldn't have! She was alive! I felt her! She was alive! It's impossible! No!!!
So at this point Vader believes he has unintentionally killed Padme, and by implication her children, and his children.
In Episode V we see:
VADER: What is thy bidding, my master?
EMPEROR: There is a great disturbance in the Force.
VADER: I have felt it.
EMPEROR: We have a new enemy - Luke Skywalker.
VADER: Yes, my master.
EMPEROR: He could destroy us.
VADER: He's just a boy. Obi-Wan can no longer help him.
EMPEROR: The Force is strong with him. The son of Skywalker must not become a Jedi.
VADER: If he could be turned, he would become a powerful ally.
EMPEROR: Yes. Yes. He would be a great asset. Can it be done?
VADER: He will join us or die, my master.
So we see Vader realizes that his son is alive, and by implication, that he was lied to by Palpatine.
My question is: When does Darth Vader realise Palpatine lied to him about his killing Padme?