Usually. Research shows that on average vegetarians consume 500 less calories a day than meat-eaters, and this will lead to weight loss, but it depends on the details of your diet. If you eat lots of french fries, sodas & ice cream you might be vegetarian but won't lose weight. Eating lots of vegies and limiting sugars and grains will help. See http://weightloss.About.Com/cs/nutrition/a/aa092903a.Htm.
Yes. Weight is calories-in, calories-out. Vegetables taste great without the calories; and the processed foods that the rest of us gorge on are so yummy that we get fat off them. Longstanding studies of vegetarians show little or no overall improved survival or decreased risk of any of the major diseases, but many people find it physically and spiritually more satisfying. Your choice.