We have done a fairly large number of fleet graphics designs (and contracted the actual installation of the wraps).
We've done several vans, a box truck, a half dozen trucks with roll-up side doors, and a couple of 18-wheeler trailers (and their cabs).
Our art director already has the templates in place for just about every make/model/year vehicle there is out there... which is helpful as exact measurement of all the body panels is critical, especially for continuous designs, etc. She's really really good at it, if I do say so myself.
Vehicle wraps vary wildly... you'll find great design and poor design, great installation and terrible installation, and extremely high prices and some much more affordable. It just depends on who you use.
Contact me off list (contact info in profile) if you'd like to talk about it.
Fantastic Plastic Entertainment, Inc. fantasticplastic.com
Speaking of vans, I hired a company last year that had plain white cargo vans, but remote controlled hydraulic panels on the sides for their tripods, tape stock and lighting gear, plus the padded drawers in the back for the cameras.
I'm with Biscardi on this one. Unless you plan to have someone with the van at all times, you're just asking for all kinds of trouble. But a good compromise might be to go with removeable magnetic signs instead of vinyl. Best of both worlds; flash for the relatively few times when you need that, stealth when you don't want trouble.
I would rather spend a little on custom shirts or uniforms than graphics for the van, if I was going in that general direction. When you see a staffer in a logo shirt, you may strike up a conversation and they can act as a sales contact. ("Oh, I see you work for so-and-so?" "Yes, do you have a question about our service?...." ) The van can't speak or answer immediate questions and hold conversations like a person can, its just a billboard.