As said before they get brittle with age,I replaced all of mine with the Polycarbonate ones then put
Maxxair covers over them,been there more than 10 years and are still good as new.
If you are looking for vent lids.First see what type vent lid you have using this

For long lasting vent covers
Polycarbonate ones are the ones you want.
If you haven't done this before
Here is a short video that will help you.
You may need a new seal that goes on the metal edge this will make it a tight seal
This is what you needReplace like this