Can a taco vendor (taquero) come to the park for a private party?

Call them

more than likely you have to go thru La county parks and recreation's or may be a state park . basically go down to the city of Lancaster office and ask for a step by step how to file for a permit for a vendor event / private party .

I'm not 100% sure, You should call them & ask them just to make sure...

Thank you. I will call.