If you are underweight, you need to eat protein. It is probably unwise to go vegetarian unless you can eat LOTS of vegetables, and you include vegetables like spinach, broccoli, and fatty veggies like avocados. Also, read up on vegetarianism...its a trend, I know. But the reality is that humans are NOT vegetarians by nature, and that becoming one is unnatural for humans.
Q: What is a Vegetarian?
A: A poor hunter
Humans and all primates are hunter gatherers, and that has been the driving force of our migrations as well as the first territorial conflicts amongst humans. Hunters arent hunting plants.
There was never a tool found by early man which was made to cut plants.