Just because it is white and stamped PVC doesn't make it all the same. Interesting to read all the different rates of UV degradation seen on this thread.

Being a budget sailboat racer for many years, I used PVC pipe as a wisker pole on a 25.5' Hunter. The pole was 9' and stored on the deck in direct sun. I used the pole for over 5 years and it was flexed frequently without cracking or breaking. I also made a bottom brush of 4' sections and threaded fittings, one was a 45 deg. elbow to reach the bottom. The pipe was mfg. by Charlotte Pipe which may have a better than normal UV formula, don't know.

Just my experience with PVC, don't think I have a problem with vents that are not painted.