I've only been vegetarian for a short time, and so in some ways I don't feel qualified to comment, so keep that in mind. However I have recently lost some weight on a vegetarian diet in the last month or two (around 7 kilos or nearly 10% of my body weight), and am currently reading a book on nutrition which I've almost finished. It's called Becoming Vegan It even has a specific chapter on vegan diet while losing weight.
Here's my advice, taken from a combination of this book, other books and articles, common sense and personal experience.
1. Focusing on eating less food overall rather than just what you eat. Reduce snacks. E.g. I eat a small lunch at 1pm. If I get hungry say 2.30pm I eat a snack, but it should be more often fruit than junk. If I get hungry after 4pm, I just go hungry until 6pm when I have dinner. Eat until not hungry, not until stuffed full. If you are never hungry or always eat as soon as you are hungry, then you are eating too much.
2. Mostly replace meat with extra fruit and vegetables and legumes, not with extra carbs or junk food.
3. Minimize processed food, as well as being unhealthy they contain things like sugar, salt and monosodium glutamate that are addictive and make you eat more, even after you are full.