The problem with vinyl windows is that they're so soft that they easily scratched by even dirt on their surface if they're rolled up vinyl-to-vinyl. They're so soft that using paper towels on them will eventually absolutely ruin them. The best you can do is what Stinger suggested... clean them as best you can before storing them while you're offroad. Getting the dirt off first will do a lot to keeping them clear. And to be sure, make sure you never use paper towels on them. Old t-shirts, old cloth diapers, microfiber cloth, etc. are fine but never any paper products since those are made from wood pulp.