I will not impose on you my point of view about vegetarianism, but you will not lose weight or gain weight on greenery. Rather, your muscles will weaken.
In general, I advise everyone to a proven method: a protein diet. Fat will decrease, muscles will increase. Drink more, less carbs, and more exercise.
In general, the best diet is sports. And that's it. The main thing is not to reach the jock-ups.