If you want to continue being a vegetarianthere are ways to reduce your caloric intake or increase your caloric burn rate without eating slaughtered animals.
It's more than calorie in/ calorie out. If you are malnourished, you will continue to crave food. You need to get enough vitamins, minerals, and protein or you will still be hungry. When people are hungry and eat junk food, that is often when excess calories are consumed; and they are not satisfied until they get the nutrients they need.
Talk with a health care professional before dieting. Better than dieting; eat healthier.
If you are a devoted vegetarian you would keep to vegetarian food. Otherwise ask your doctor.
Instead of meat, you can eat beans and bean products, nuts and seeds, whole grains, and (if you like) dairy products and eggs. Cut the refined carbohydrates; my observation is that overweight people are sugar junkies.
You should ask your doctor before dieting. But it all depends on how serious of a vegetarian you are. Try tofu it's actually really good.
You can successfully diet and continue being a vegetarian. If you are dieting to lose weight, check Dr. Ornish's program, Brenda Davis's books, nutritionMD online. You will find guidelines for selection of foods to create a balanced diet, and hundreds of easy healthy recipes.
No, you should not allow such things as fish, game, meats (including chicken, etc) back into your diet if you are vegetarian and especially if you became a vegetarian on the grounds of principle. If you are having difficulty losing weight, or if you have reached a plateau, engage in cardio exercise (or engage in more cardio exercise). Cardio is an Excellent fat burner. If you are new to exercise simply begin with cardio walking. It is easier on your body than other forms of cardio such as running or jogging and it is sustainable long term. In addition, if you are able, engage in weigh training to increase your muscle mass a little (don't worry you do not need to build huge muscles). The more muscle we have the more calories we burn. Muscle burns calories 24 hours per day and not just when you are exercising.