Here's a good discussion of the nutritional risks and benefits of a vegetarian diet for athletes: Fueling the Vegetarian (Vegan) Athlete. It includes a list of suggested supplements.
Vegetarian diets are associated with several health benefits, but whether a vegetarian or vegan diet is beneficial for athletic performance has not yet been defined. Based on the evidence in the literature that diets high in unrefined plant foods are associated with beneficial effects on overall health, lifespan, immune function, and cardiovascular health, such diets likely would promote improved athletic performance as well. In this article, we review the state of the literature on vegetarian diets and athletic performance, discuss prevention of potential micronutrient deficiencies that may occur in the vegan athlete, and provide strategies on meeting the enhanced caloric and protein needs of an athlete with a plant-based diet.