He doesn't need people worrying for him - he can take care of himself. [Written for Ironman7's painfic week.]
But he doesn't want her to worry. He doesn't need people worrying for him - he can take care of himself.
Which is why he's on the bridge of the Strahl, leaning over Balthier's shoulder and exchanging insults in the middle of the night while all the others are asleep. He doesn't want to think about - well, he doesn't want to think about anything, and judging by the dark circles under Balthier's eyes, he doesn't either. So they sit and argue until one of them falls asleep, and Vaan thinks he likes it better this way.
Balthier doesn't worry about Vaan, and he'd laugh if anyone asked him about it. He's got his problems, Vaan's got his, and they get on fine without sharing them.