It'd be difficult for a cat to survive on a purely vegetarian diet, forn the reason Troll gave (unlike a dog in this respect).

There's a tiger sanctuary in Thailand run by Buddhist monks where they give the tigers' meat that's been prepared in such a way that it doesn't look like meat, or taste like meat, but they still like the taste. The idea isn't just that the monks are being sqeamish about meat - it reduces the chance of a tiger accidentally twigging that people are made of the same kind of stuff they eat.