As Glenn wrote, Vader knew about Luke before that conversation with the Emperor. Near the beginning of Empire, Captain Piett shows Vader video of the Rebel's shield generator and when Admiral Ozzel disagrees that it could be anyone, Vader mentions Skywalker:

VADER: You found something?

PIETT: Yes, my lord.

VADER: That's it. The Rebels are there.

OZZEL: My lord, there are so many uncharted settlements. It could be

smugglers, it could be...

VADER: That is the system. And I'm sure Skywalker is with them. Set your course for the Hoth system. General Veers, prepare your men.