I have little personal experience with propane heaters in a van. But, here are some thoughts.
All hydro-carbons, wood, gasoline, propane, butane, paper, are composed of hydrogen and carbon. When they burns, the carbon in the fuel and oxygen in the air make carbon dioxide and perhaps carbon monoxide. The hydrogen in the fuel and oxygen in the air produce hydrogen dioxide, commonly known as water. If you burn pure hydrogen, you get no carbon dioxide, only water. This also produces heat.
The unvented heaters we're talking about heat the air. The water vapor produced goes into the air.
If you open a window, some of the heated, moistened, air will escape, and be replaced with air form outside the van. But, this air is also taking the heat with it. I think you'll accomplish the same result by running the heater less and keeping the window closed. It will produce less heat, but also less water vapor. You can't vent the water vapor without simultaneously venting the heat. So what's the point.
Just a thought.