When Leia, Chewbacca, and Han confront Lando about them being tortured, Lando says that Vader is not even interested in them. He's after somebody named Skywalker.
Leia: Why are they doing this?
Han: They never even asked me any questions.
Lando enters with guards.
Chewbacca growls even louder and angrier, directed at Lando.
Han: Get out of here, Lan ...
Lando: Shut up and listen! Now Vader's agreed to turn Leia and Chewy over to me.
Han: Over to you.
Lando: They'll have to stay here, but at least they'll be safe.
Leia: What about Han?
Lando: Vader's giving him to the bounty hunter.
Leia: Vader wants us all de ...
Lando: He doesn't want you at all. He's after somebody called ... uh ... Skywalker.
How did Lando know Vader wanted Luke?
Did Vader let that slip? Did Lando get that knowledge from other sources? It's not like anybody else with Vader (e.g. - stormtroopers) know about Vader's quest for Luke.
Is this mentioned in the novelization?