Can you tell me how much weight will I lose switching to a vegetarian diet and consuming 300-600 cals?

Depends . I'm not sure if you mean you will only be taking 300-600 cal of vegetarian food for the day which is too low of a caloric intake and not recommended on any diet. The minimum caloric requirement on a very low calorie diet is about 800 cal, mostly from proteins and fat. That will result in at least 2 to 3 pounds of fat loss a week in a ketogenic state.