All of the restaurants will have vegetables at least as side dishes. You'd need to talk to the waiter to find out how they're prepared (sometimes you'll find veggies in a beef broth. for example). There are speciality dining restaurants on NCL that are Asian -- and you might well find a wider variety of vegetable dishes there. And there's always the buffet where you can pick and choose. I've seen people have conversations with the waiters, which lead to conversations with the chef, and they'll be presented with a lovely plate of some special dish prepared for them, so it probably won't hurt to ask, too!