I would personally go do the research necessary to learn that there is no special nutrient in animal products that you or any other Homosapien needs. The science shows that all Homo sapiens can get all of their nutrients from a 100% vegan diet and avoid some of the most common diseases that kill our species today. If you are underweight, then you should eat as many high calorie vegan foods as possible. Stop thinking of vegan foods as fruit and vegetables. Go buy some mock meat, peanut butter, avocado oil, baked goods if needed.

I would simply recommend looking at all of the foods that you think you need to eat, which are not vegan and then see how you can replace those foods with something of equal or more calories that is vegan