I've seen vehicle signs used most effectively for businesses that make house calls. The simple fact that a company vehicle has shown up at their house instead of an unmarked van will look more professional. While you do not want to put too much information on your vehicle, displaying your company name and logo will allow potential customers to recognize you, and your company will be the first one that comes to mind when they need your services. Company recognition is one of the most useful advertising tactics, and your vehicle can make that happen.
Truck Magnets are a great way to use a personal vehicle as a work vehicle. These magnets can be moved from one vehicle to another as many times as needed, without leaving any residue. Great for food delivery vehicles!