IRS Pub946 covers depreciation https://www.irs.gov/forms-pubs/about-publication-946
The food truck would be eligible for Sec179 in the year purchased. The totality of the return would determine if allowed. Same would apply to van.
If the food truck is licensed, and can be driven, it should be reported on an asset worksheet, as you will have ongoing expenses that you want to track. I would think you would have low miles, so you would probably want to take expenses rather than mileage each year, but if you do a lot of travel going to fairs, and such, the first year you may want to take mileage. After the first year you will have a better idea on the kind of mileage you are going to put on. The reason for that is, you can't change to mileage, unless you took mileage the first year
You'll want enter on Car and Truck Expenses Worksheet. There you'll be able to enter mileage to support the 100% business use. Also you can elect to deduct actual expenses or standard mileage on that worksheet. As @Terry53029 pointed out, you may want to consider what's the best option based on how much mileage will be. Sec179 may give you nice write off the first year but how will actual look compared to mileage in future years?