While diners often serve great food, they're not the best place for you if you're a vegetarian. Your best bet at a diner would be to stick to salads. If there are other vegetarian dishes on the menu, be cautious and ask the wait person how they're cooked. You might find that they're prepared with meat products to add flavor.
Sadly, a diner isn't the best place to go if you're trying to lose weight. Even if the offerings seem to fit your diet plan, keep in mind the fact that the dishes might not be prepared in the healthiest ways. Often, diner fare is heavier and greasier than most special diets allow.