Hi quizmaster,you could try vinegar and baking soda.mix one cup of vinegar with one teaspoon of baking soda,use a rag in a circular motion.do not soak the carpet and dry with any dry cloth.When animals are sick,it sometimes has the same effect as bleach and you wont be able to remove it.The trick is to remove it as soon as possible...good luck.
I would try your cat on heroine instead of grass. It worked for my cat. He isn't sick anymore. Just lies about on the couch with his headphones on, listening to Groove Armada. Hope this helps.