Subtlety has never been his style. For Han if you mean to say something, it's best just to say it and be done with it. Leia, of course she's diplomatic when she needs to be, but she's never been afraid to speak her mind to him. Maybe that's partly because she's kept him so long at arm's length; there couldn't be much fear of driving him off when that's what she was intending to do, but he's stuck around in spite of everything. By now, he figures, she's got to know he's not going anywhere.

Which is the only reason he doesn't snap back the obvious accusation that she must be joking. Because it's utterly ridiculous; Luke is about the furthest you could get from someone like Vader. The only resemblance is a total inversion.

He opens his mouth, and then shuts it again, looking at her like he hasn't seen her until now. (Really, he does that a lot, if for different reasons. If he lives to be a hundred he'll never be tired of looking at her, he's sure of it. If she lets him keep doing it that long.)

"Are you sure?"

Maybe that's a stupid question, but he can't help doubting it.