How do I restore the whiteness to old vinyl windows?

When that doesn't work, try soft scrub with bleach.

https://homeguides.sfgate.com/brighten-vinyl-windows-dulled-28042.html

Hi Sac,

You can try any of the items you mentioned. For toothpaste, you want a white one, not a gel. Just rub it on the frame and let it sit a while before removing and rinsing. Baking soda can be made into a paste with water. Put that on, again, let it sit a while, then spray with a 50/50 mix of vinegar & water. Rinse and dry. You can also make a paste with baking soda and hydrogen peroxide, put that on, let it sit then wipe and rinse. None of these need an exact recipe. You'll see the paste forming with the water or hydrogen peroxide and baking soda. I use these all the time and have found that the baking soda and hydrogen peroxide works quite well. If it doesn't work completely, try again. If it won't fix the problem, you might have to paint them. Wishing you the best.

Use the Magic Eraser sponges!