Look at the online menus. If there is something that may be possibly prepared vegan, you should be ok with just showing up. Other locations that don't have much of a veggie friendly selection (Bisto de Paris I'm looking at you... and your plate of side dishes....) call Disney Dining and have them add it to the reservation. Online can't handle it, but the phone system can. Some places like Hoop De Doo Review have limited goods in their pantry, so really need advance notice unless you want to end up eating PB&J.