Protein's important for gaining and maintaining weight too, and that can be harder to get for a vegetarian but not impossible of course. I'm sure you have your ways.
Some years back a vegan posted some details about her gain. I don't remember much but I remember she was consuming a lot of cooking oil! It's very fattening, about 120 calories per teaspoon.
I'd mix it into your pasta or veggies or whatever and gradually increase the amount you use. It can be pretty hard on the digestive system if you take it straight.