To complete this mission easily, 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 [Fire] for two seconds. 
The vans will explode very quickly.