I try to avoid vending events like the plague! You can either make a pile of money quick, or you can lose your arse, it is a total crapshoot.
Understand that the main job of the event organizer is to sell real estate at the event, they may inflate attendance numbers, they may have 15 booths selling the same thing, the spot rental may be way out of line. Weather can and will affect sales in a huge manner.
See how many other food vendors are there, see what they are selling, and figure about 12% of people in attendance may get food there, and divide that number by the number of vendors to get an idea of your sales.
Then look at your health department inspection/licensing fees, if the organizer wants a percentage of your gross sales, figure loss at your restaurant and labor and see if it is worth the chance.
Then there is logistics, transport, delivery, mileage, maintaining quality. Etc.
if you decide to do it, I wish you luck.