Island Lawn, I would not worry about doing anything to the van. If this lady is a professional she will know how to prep the surface. They will most likely wash the area with soap and water and then wipe it down with a wax and grease remover. Then use a solution formulated to ease installation of the vinyl so they can squeegee the air bubbles out.
When I took my trailer to get done last month there was more snow and salt sprayed on it than you could shake a stick at. They know how to get that stuff ready, let them do it! Good luck!