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My advice would be to get more proteins and fats in your diet. Vegetarian diets consist of mostly plants (of course) that are mainly made up of carbohydrates. Make sure you are getting plenty of proteins and good, healthy fats. Many "classic" carb-rich, vegetarian combination of foods provide all of the essential amino acids that your body needs to construct proteins, called complete proteins. Do a search for "complete protein food combination" and you should see how the pattern works. It usually involves putting some type of grain product with another food. Examples include beans and rice, peanut butter and bread, etc.
Eggs are also a complete protein in themselves and contain fat, too. Also make sure you are getting plenty of good fats, like those in avocados, olives, olive oil, nuts, seeds, etc. These add calories and are easy to add to things (plus they taste really good).
Also, it would also be good to get a check-up by your doctor to make sure that there isn't some medical reason for you not gaining weight. Your doctor may also suggest that you have a consultation with a nutritionist to make sure that you are getting everything your body needs, while still maintaining your vegetarian diet.
Do some searches on your own. Try also search for "gaining weight as a vegetarian."