They have vegetarian options on most of the menus by default. I'm not sure if you ordered a veggie meal that they'd load anything different or just load more of the veggie items already on the menu. If you have a special meal they will usually come to you when everyone else is having their options taken to explain what they'll serve you. There is no later meal service, as you say it's dine on demand - so if you want it, ask for it. You might find they're able to accommodate requests but best to ask onboard.