Having sailed with Disney Cruise Lines, multiple times, I pulled out a menu from one of the cruises, and these were the vegetarian offerings for that evening's dinner.

Entree
Roasted Portobello Triangoli Pasta
Vegetable Wellington
Asiago and Asparagus Risotto

And there was always two or three different vegetarian entrees each evening for dinner. But, if those don't tempt your palate, you can stick with the appetizer, soup, and salad. At least one each night being vegetarian.

Appetizers
Sundried Tomato Spinach and Feta Cheese Galantine
Golden Pineapple Cristobal

Soups
Chilled Apple and Watermelon Gazpacho

Salads
Gourmet Mix of Salad Leaves

As for the buffet restaurant, I never ate there for dinner, always eating in the dining room, but for lunch they always had a choice of vegetarian salads, soups, entrees, etc.

And even the snackbars up on deck had your choice of vegetarian offerings.

Whenever you cruise, enjoy your cruise.