What would you use to wash a vehicle that had been exposed to hydrochloric acid?

You would want to neutralize the acid by making a solution of sodium bicarbonate(baking soda) or by dusting the vehicles with Straight baking soda athen hosing it off. at least just rinsing off the hydrochloric with plenty of water would in some cases be sufficient. If the acid has had time to react with the paint then the pigment has chemically reacted and you probably can't reverse the process

wash with solution of  bicarb of soda & H 2 O

There are specific acid neutralization solutions you can buy from chemical retailers such as vwr.com.  But really, just hit them with lots of water and that will do the trick.

But if you do want to be thorough, there is no need to spend a lot of money.  For a do-it-yourself neutralization wash, mix a 1 box of baking soda with about every 4-6 gallons of water, and wash with that.  The baking soda is primarily sodium carbonate and sodium bicarbonate.  When reacting with acid, the carbonate and bicarb will combine with the acid to form carbonic acid (H2CO3) which decomposes almost immediately to form carbon dioxide and water.  

There is really no risk involved in washing, but washers should wear goggles and gloves, and try to avoid direct splashes off the surface of the truck, as this could contain a bit of unreacted acid and cause burns.  If you get burnt, just wash with a lot of water.   The end result is no different than getting burnt with an iron, and completely avoidable if you wash it off quickly.

Hope that info helps!  Cheers~

Edit:  I should probably note, if significant quantities of acid are on the truck, some fizzing or bubbling of the baking soda solution is to be expected.  Basicly just like you would see if you pour a baking soda solution on the battery terminals in your car to remove the corrosion...