Your vet is a nitwit.
http://www.felinefuture.com/nutrition/bpo_ch8a.php
Although vitamin supplements are sold for cats and dogs, I challenge you to find a veterinary organization that recommends vitamin C supplements for housecats.
I have two 16 year old housecats that have never had anything but Cat Chow.
Her diagnosis must have been accurate, since her treatment was effective.