I went to the game on Sat with a vegetarian who enjoyed 9 veggie dogs (and the beauty of dollar dog night) on a dare. I didn't see where he got them, but they came in the same type wrapper (with the word vegan on it) as the regular dogs I got from a plain old hot dog stand.
There are restaurants in addition to the stands - Harry the K, in and Up, etc. I have had a decent Tuna steak sadwich in the Hall of Fame Club. I suggest you call the Phillies. . I went last night. Enjoy the game