To easily complete this mission, first get a police car, and log onto the police computer. Search for an arson crime (fire making criminals). Do the Vigilante mission as usual, but when you kill the criminals, pick up the flame-throwers. Start the "Store Wars" mission, and quickly park two cars at each end so it makes it difficult for the vans to get past. Let the vans drive towards you (or you go towards them). Equip the flame thrower, and hold A for approximately two seconds. The vans will explode very quickly.