If you want an animal that is going to be dedicated to killing rodents, I think a Jack Russel or similar breed is a better bet than a cat. My mutt is very predatory and will spend all day killing rodents outside. Bring her in and she lays on the couch and ignores the mice I had to trap. She killed two when we first moved here, and then just stopped caring about indoor rodents. I've always adopted, but you never know what you will get with a mixed breed. I'm in a new area, but I never had a problem with voles bothering my potatoes or other veggies when I lived in NC with clay soil. I didn't have a fence or other protection, and before my dog there were a lots of rabbits and groundhogs too; but my garden was free from all small mammal pests after I brought her home and she wasn't even outside all the time. I used to put the sweepings from inside the house in the garden too.
You can buy predator pee and try that as a deterrent. There is coyote, fox, bobcat, and mixes available to help scare rodents and rabbits away.
I have worked on two farms that swore by Jack Russel dogs for rodent control. Cats can be useful, but they are not usually as obsessive about it as a dog bred for this purpose. These dogs will want to do nothing else, and if you can deal with the damage from their digging, they will kill, kill, kill whether they are hungry or not, and they never get bored with it. They will cry by the door for you to let them out to hunt.
You can contact a Jack Russel rescue and they often know if there is a predatory one available in a foster home. I know one of the farms had adopted an adult Jack that was given up for being too predatory. The other farm had sought out a breeder that produced dogs for this trait, and participated in earth dog trials. They raised the puppy around chickens and cats, so the dog didn't bother those. If you brought this dog by the pond he would hunt bubbles in it. Their hunting is an obsessive compulsive disorder.
My dog is not quite as dedicated, but she seemed effective enough in our past location.