Fortunately for most vegetarians their diets tend to be healthy, obviously low in saturated animal fats, and high in natural plant matter.
Unfortunately due to the fact that most vegetarian diets are healthy, finding one to enable you to lose weight will be hard to find.
Vegetarians should look at the foods that they eat, to find out which foods are high in calories, this will tend to be nuts, cheeses and various oils that food will be either cooked in or added as a side dressing.
There are now plenty of good books for vegetarian cuisine, look at trying new and different foods in order to extract the various minerals and vitamins that your body will require. Most vegetarians, especially those looking at reducing their calorie intake will be deficient in the trace elements that the body will require in order to function at its full potential.
I would strongly suggest supplements in the form of vitamins and minerals, look at boosting the ones that you are lacking through your diet, i.e. iron. Combine this with regular aerobic exercise and body conditioning to both increase and promote active weight loss.