Veggie Happy keeps an updated list of veggie options in baseball stadiums (as well as other types of stadiums). For example, Dodger Stadium in Los Angeles is now offering a Tempeh Basebowl and Tempeh Nachos with vegan cheese. Target Field in Minneapolis offers Vegan Sriracha Brat, Vegan Italian Sausage, Vegan Tofu Vindaloo, and more. Wrigley Field in Boston sells Spicy Tofu Tacos with Napa Slaw and Lime Crema and a Roasted Cauliflower Sandwich with red pepper pesto and chickpea spread.
Visit: https://www.veggiehappy.com/listings/?listing_categories=mlb and you can see if a stadium near you offers vegan options.