To be honest your question is kinda vague so hard to answer exactly. Saying that I vend every Thursday at our Farmers Market in our town, and when not competing I also vend at different events in our area.
First what are the requirements in the area you are looking to vend at. Insurance , bonding, security background checks, licenses like peddlers license etc.. Then you have the health department requirements, while it is all the State of Iowa some counties are harder than others. Then you have your food liability insurance to think about, while it is not expensive you will need it. You will need a Safeserv certification, but I think you have a year to get that. Then commercial insurance for your trailer or food truck. Commercial vehicle insurance is not overly expensive but a lot of insurance companies wont touch a food truck/trailer. So sometimes a bit of leg work. Make sure you have the right ways to keep things hot and cold.
Now saying that you dont have to go into major debt to start doing this. My wife got laid off from the school last year and we needed something to supplement our income. We had a 27ft camper that we stripped out and made a vending trailer out of. Has to be able to scrub walls, no carpet, no beds, no bare wood. Our little community has nothing, so for the town my only requirement is a $15 year peddlers license. Smokers do not have to be onsite, but everything needs to be prepared on our smoker or in the licensed trailer.
I am trying to remember everything as I am prepping for cooking tonight so I may be missing something but there should be a start.