Here is the scene where Lord Vader returns to Coruscant and has an exchange with his master, the Emperor. It takes place right after Luke returns victoriously to the Rebel base and before the celebration.
A huge Star Destroyer comes out of hyperspace and heads toward the planet Coruscant. An Imperial shuttle flies out of the hangar and enters the planet's atmosphere.
The shuttle races toward the government center of Coruscant, where it is now nighttime. The Imperial shuttle flies toward the towering Imperial Palace, which is an ostentatious display of the Emperor's power. The Palace makes the nearby Senate building small by comparison.
The shuttle flies into the grim hangar. The shuttle lands right before him and the ramp lowers. Lord Vader appears and walks down the ramp. Once off of the ramp, he kneels before his master. The Emperor, however, is not happy.
"Rise, Lord Vader," he says, not pleased.
Vader stands up. "Master, the Death Star has been destroyed," he says.
"I know," the Emperor replies. "I am most disappointed with you, my apprentice. The Rebels should all be dead by now. Why aren't they!"
"There was a flaw in the design that the Rebels took advantage of. Grand Moff Tarkin overlooked it."
"Well, he's dead now," the Emperor says, smiling. "A just reward for his stupidity. And for the Rebels on Yavin 4, we shall extract revenge. They will pay for this outrage."
The Emperor and Lord Vader slowly walk together toward the exit of the hangar.
"Master, there's one more thing," Vader says as they walk. "The flaw was so minor that the Rebels should never have been able to exploit it."
"Then what happened?"
"The pilot who destroyed the Death Star, the Force was with him."
The Emperor stops walking, and Vader follows suit. The Emperor slowly turns around to look at his apprentice and stares at his black mask.
"How is that possible?" the Emperor asks. "The Jedi order is no more. You hunted down and killed all Jedi and Force users. You just got rid of Obi-Wan Kenobi. How are there any left?"
"I don't know."
The Emperor slightly nods his head and turns around to face the hangar exit. He begins walking toward it again and Vader follows by his master's side.
As the Sith exit the hangar, the Emperor says to Vader, "We will look into this matter further."
Then, we cut to the celebration on Yavin 4, which has a different feel knowing that Vader has reunited with his master and that they plan to strike back. For Episode V, I'd have Vader have the same conversation with the Emperor (but without the whole "I have no doubt this boy is the offspring of Anakin Skywalker." "How is that possible?" thing).