Vegetarian diets can be nutritionally adequate for athletes as long as you carefully plan meals that provide the right nutrients for your active needs. On average, carbs will take up half of a good athletic diet with the rest split between fat and protein. Carbohydrates can be found in vegetables, fruits, beans, and grains while protein can be found in beans. Some vegetarians may add dairy and eggs, although others prefer not to eat them. Planning meals with proper nutrition in mind and stocking your kitchen with staples will help you prepare vegetarian meals for athletes. Here is a paper that discusses practical advice for athletes that want to follow a vegan diet.
Vegan Diets: Practical Advice for Athletes and Exercisers: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5598028/