Some animals are natural born predators get over it!
Seriously get over it. As much as we need to be kind to animals and have the intellect and spiritual responsibility to minimize our impact on their habitats and territory --- animals don't have the same set of ethical responsibility or conscience we do. Their world is crude bestial and sometimes quite brutal with no shame of the consequences.
It seems like a bad idea to force a cat or dog to be vegetarians, They are evolved to be predators in the ecosystem. They have their own rightful and natural place doing so. Cats are quite happy content and free and love their place hunting birds and mice. They're lazy and weak indoors domesticated eating processed food.
If you really want to have vegetarian pets get some pet who is evolved to be vegetarian
horses, goats ,ponies, donkeys, sheep, geese . .. etcetera
That's really the best and only way to be the complete ethical vegan who claim you are, otherwise you're taking a predator animal as a pet who is built to digest meat as a staple part of his body and denying him is role in culling the herds of rodents and other vermin.
Sure some animals are natural born predators...but domestic cats aren't even naturally born...its all our fault. Guess what??? Killing for food is natural too...even for naturally vegan animals...should we do that too???