I'm surprised you haven't had any responses to this post. Another way to put this is hiding in plain sight. I read a post on another Blog by someone living in Europe. He slept in his car in areas of heavy traffic and said that was a great way to be stealthy.
I wouldn't park in a wealthy neighborhood no matter how new and nice my van was, but I maybe that would work. Some rich people who don't work have too much time on their hands and are quick to report anything that upsets their delicate world. I would consider parking in a well-lit parking lot with lots of traffic instead of in the shadows. I would maybe blatantly park in the parking lot of a restaurant across from a city police department. That is certainly counter-intuitive, but maybe that method would work. I don't have any real world experience, but I imagine if you park somewhere and make it obvious and with an air of purpose, you will be less likely to be "noticed".
While this is a bit off topic,another thing that comes to mind is a phenomenon called "Inattentional Blindness". You often hear of this regarding motorcyclists. Many times when a car has struck a motorcycle, the motorist will say he/she didn't see it despite the motorcycle obviously being right in his/her line of vision. If the brain is trained to see cars and not motorcycles, the brain will disregard motorcycles and not "see" them.
I had the same thing happen to me when I drove bus. Many, many times drivers of cars didn't "see" me despite looking directly at me. Then when they looked again, they got quite a shock watching a 30,000 pound, 10-foot tall bus coming right at them. They didn't see me because the bus wasn't a car.
I wonder how inattentional blindness could be applied toward vandwelling. If a vehicle is not in the right spot, would certain vehicles less likely be "seen" even though the vehicle is in plain sight? Would a garbage truck attract less attention despite its huge size and smell? Would pastels or certain combinations of colors work better? Or would a Day Glo Green color be so obvious that people would think, "Hmmm. No burglar/vagrant would drive such an obvious vehicle! I'm not going to say anything."
I know you were talking more about places, but many times how a van looks in context with a specific place makes a difference.