You can put the deleteVehicle line in the activation field, but take care. If you are also calling a script that needs the trigger name or location (say to cause an explosion) then the trigger will have gone by the time the script gets to it.
When I delete a trigger in this way I always put it as the last instruction in the activaion field, so that all the other instructions are run before it is deleted.
To delete a trigger you must give it a name. this doesn't work the way you expect with triggers
Here is an activation field I made earlier (the trigger is called trig_Scene_Vigny):
[] exec "Scene_Vigny.sqs";deleteVehicle trig_Scene_Vigny