Have you contacted the Veg Festival organisers? I think their is a Vegan Fest too somewhere.
There is a Vegan Society too!
I strongly suspect tall these have a list of preferred and selected traders on their books and in your area?
Asian style foods are very popular with event organisers but tend to be more adult friendly, whilst those selling flatbreads, wraps, pizzas etc suit all ages.
NCASS also have a service to find suitable traders - all will have been checked for the documentation required of street food traders.