Krylon Fusion for Plastic (spray paint). Bonds directly to hard vinyl; they say it can be used to paint vinyl patio furniture. I would expect patio furniture to take more abuse than window frames, so this paint might be durable enough for your application.

http://www.krylon.com/main/product_template.cfm?levelid=5&sub_levelid=10&productid=1751&content=product_details

__________________

1989 250TD Wagon 5-speed, 160,000mi ::: Dark gray metallic / black cloth
1984 190D-2.2 5-speed, 287,000mi ::: Silver-blue metallic / black MB-tex ::: SOLD