Vegetarians have many options if they pack their own lunches. Here are some suggestions:

Pasta salad
Hummus with veggies and pita
English muffin pizza
Bagel and cream cheese
Soup (in a thermos): tomato, black bean, lentil, vegetable…
Vegetable sushi
Peanut butter and banana or jelly sandwich
Bean burrito
Pasta dishes in a thermos (spaghetti, mac n cheese,etc.)
Chili (in a thermos) with a baked potato in foil to pour it over.
Falafel sandwich
Tortilla chips with a Tupperware bowl layered with salsa, refried beans, guacamole and cheese and olives to scoop up
Whole grain pita with vegetarian re-fried beans, organic avocados, and salsa. Add cheese if you like.