I've been looking into it too, the answer is really to decide where you want to vend, find out who has jurisdiction there, and talk to the health department for that jurisdiction.
I live on the edge of Arlington and Fort Worth. If I want to sell in Arlington I have to deal with the Arlington HD. In Ft. Worth, I have to deal with the Ft. Worth HD. Outside those two, it's Tarrant County HD. With all the little towns and such sprinkled right in the middle of the DFW area, I can have to deal with three different health inspectors driving 10 minutes from my house, just depends which direction I go.
In any case, a SF pit may not the right tool for the job. It wouldn't be around me. Most of the health departments around me specifically want a NSF-certified cooker, so you're talking Ole Hickory or something. And at least around me, you can only cook on-site if you're working under a temporary event permit. If you're running a food truck or something, the actual smoking has to happen in a licensed kitchen. Again, talk to whatever health inspector would be in charge of the spot where you want to vend to find out what's legal and what's not. Austin is different from DFW. Houston is different from Austin.