One thing that I haven't seen mentioned in any of these posts is the role that the hardener plays in two stage systems. Using the wrong ratio of hardener or too fast of reducer can cause the brittleness that you have experienced. I used to tint windows with car paint that holds up as well or better than vinyl, its a matter of getting the ratios right. Experiencing chipping (brittleness) is likely caused by poor prep work or a failure in somewhere in the application process. (Helpful, right?) If you are confident in your prep work examine the products you are using and your process. Make sure you are using the right products as recommended by the manufacturer. Also in the world of paint, cost is well correlated with quality products.