If you wish to lose weight, going to the extreme of going on a vegetarian diet will most likely cause your diet to backfire. Changing a diet drastically is a sure fire way to fail on a diet to lose weight.
When one diets to lose weight, they are forfeiting a lot of foods to begin with, then add to that a vegan diet, with all of it's limitations, you are setting yourself up to fail.
Why not just go on a sugar free and fat free diet and only eat fresh fruits, vegetables and lean meats and fish? You will be more apt to stay on a diet with foods you are used to eating but eliminating the processed foods and preparation methods that add calories. Just be a wise supermarket shopper and do not buy anything bad for your diet so it is not even in the house.
People who are malnourished are ones who eat an unbalanced and unhealthy diet.
If you want to lose weight, do not make drastic changes by eating a vegan diet as it is very restrictive. Go on a sensible diet of only fruit, whole grains, vegetables and lean meats and fish....you WILL lose weight. Also, go for a 20 minute walk 4 times a week and you will notice a difference in your energy levels and will lose weight. After you have lost a satisfactory amount of weight, THEN change to a vegan diet as a way of life, not a way to lose weight.
Please read a list of foods forbidden on a vegan diet. It is a very restrictive diet that means you must do a lot of your own food prep on your own because it is doubtful that you can go to a picnic, restaurant, family dinner or lunch with friends and eat one single thing served unless you bring your own food.