Gaining weight is easy -- eat more of the healthy foods you already eat (hopefully you eat healthfully -- being a vegetarian doesn't in itself suggest a healthy diet) and don't add any additional activity. In other words, eat more. Maybe add a meal. As to what foods to eat, you can easily gain weight by just eating a ton of sugary and fatty food but that wouldn't be healthful, which is why I suggest eating more of the healthy foods you already consume.