Knives isn't sure how he can trust that suggestion and Vash can probably feel his skepticism radiating even with the distance between them. But he doesn't make a sarcastic comment, and that's probably a sign of growth.

Then there's a knock at his door. He's here. Might not want to eavesdrop now, brother.

Because he is going to at least tease Vash a bit - even if he has to use his own sex life to do so.