"That's it, and I'm sure Skywalker is with them. Set your course for the Hoth system." -Vader, beginning of Empire.

"We have a new enemy: the young rebel who destroyed the Death Star. I have no doubt this boy is the offspring of Anakin Skywalker."

"How is that possible?" -Emperor Palp and Vader during the hologram scene in Empire.

Vader already knew he was chasing Luke; it even says so in the yellow lettered background blurb in the beginning of the movie. I think he was playing dumb to the Emperor, after all, Vader tries to recruit Luke to help overthrow Palpatine in the Cloud City duel, so he wouldn't want to reveal to Palpatine just how much he knows about Luke.