>> and they don't think that it would be covered under warranty as a defect..

Of course they are! Don't settle for anything less than repair or replacement.

The vents were created and sold to you for a specific purpose. They have become unable to perform this purpose through normal usage. They are a part of the vehicle that have failed pre-maturely. The design life of these vents is no less than 5 years, as specified in the bumper-to-bumper warranty.

As long as you did not abuse them in any obvious way, you are entitled to repair or replacement. That's the bottom line.

Stand firm! :)
Skadar