I occasionally get involved in organizing running races. I think it would
be a good idea to arrange for a "lunch truck" vendor to show up to
sell food to the runners after the race. How can I find qualified,
reliable lunch truck vendors in the greater Washington DC area (MD &
No. VA), and would anyone know what type of business arrangements are
typical of such arrangements if any, i.e. do I pay them to show up? or
do they pay me a % of gross sales for an exclusive right to sell at my
race? or??

A potentially stupid question: Have you tried any caterers? Like pig
roasts, barbecues, and the like?

Yes, I've looked into caterers. They typically want a large per-person
fee upfront (not surprisingly) for a set amount of food. I need the
flexibility offered by not having anyone pay up front (or having to
track payments) and having folks able to choose whatever they want
from the lunch truck type menu.

As a runner for several years, every race I've ever participated in
(from 5Ks to marathons), the race organizer has always provided free
food at the finish. Sometimes it is simply bananas and bagels,
sometimes they offer much more. I've never seen nor can imagine a race
where free food isn't offered.

Oh, we have free food. [Typically fruit, cookies, water and gatorade,
etc.] But we don't have sandwiches, hot dogs, sodas, pretzels, ice
cream, coffee, hot chocolate, or other typical lunch truck items, and
with increasing numbers of folks coming along with friends and family
it seems reasonable to offer more choices and enhance the odds that
folks hang around and socialize for a while.