vending at festivals is a total crapshoot. it depends on how many other vendors are there, how many vendors selling the same products, attendance, weather, booth placement, inspection fees, fuel, travel lodging. and then have large enough equipment to meet the volume needed to show a profit
The typical percentage I have learned is typically 5-7 percent of total attendance.
Where is your base of operations at compared to these festivals? You do have a licensed kitchen facility to work from, right? if not, where do you plan on doing prep, getting potable water and dumping grey water, taking deliveries, making dough?
What is your break-even point at? Choosing which events to vend at is key, if they do not have the potential to meet and exceed your break-even point, do not pay the real estate fees to attend. You may be better with catering private events to start, then work your way into vending.
I do catering, but I vend 1 event a year in the city I live in.