I envision the first reveal in the trench like this:
INT. MASSASSI OUTPOST - WAR ROOM
Leia and the others stand watching the projected screen.
INT. DARTH VADER'S COCKPIT
REBEL BASE VOICE
(over speaker)
His computer's off. Skywalker, you switched off your targeting computer. What's wrong?
LUKE
(over speaker)
Nothing. I'm all right.
Vader is taken by surprise, and looks out from his cockpit.
VADER
What?!
Vader's targeting computer swings around into position. Vader takes careful aim on Luke's X-wing fighter.
VADER
Skywalker! I have you now.
He pushes the fire buttons.
EXT. SURFACE OF THE DEATH STAR
The three TIE fighters move in on Luke. As Vader's center fighter unleashes a volley of laserfire, one of the TIE ships at his side is hit and explodes into flame. The two remaining ships continue to move in.
Later...
INT. VADERS MEDITATION CHAMBER
Darth Vader kneels before a giant holographic image of his master, Emperor Palpatine. (Palpatine's theme music is heard in the background).
Vader: "What is thy bidding, my master?"
Palpatine: "The young rebel who destroyed the Death Star. I have no doubt this boy is the offspring of Anakin Skywalker."
Vader: "How is that possible?"
Palpatine: "Search your feelings, Lord Vader. You know it to be true."
I really want to keep the lines of Palpatine telling Vader to search his feelings as they have a nice parallel with the big reveal at Bespin.
Luke: "No... that's not true... that's impossible..."
Vader: "Search your feelings, you know them to be true."