Domestic cats do not need vitamin supplements as long as the food that you provide them with is complete and balanced and of high quality. High quality, complete cat food contains all the vitamins that domestic cats need. And if you are not providing your cat with a high-quality cat food you are unlikely to provide her with a vitamin supplement and therefore the question in the title is redundant.
After years of reading about cat food, I believe that very few people provide their cat with vitamin supplements. They just don’t need it and you may do harm actually. Veterinarians would agree with me. I can quote from a reference book that I have: “Vitamin and mineral supplements are not necessary or desirable when you are feeding a nutritionally balanced growth food”. This sentence comes from a section on weaning kittens from their mother. And, in another section on feeding kittens, but the same would apply to feeding adult cat, the veterinarian authors of the book Cat Owner’s Home Veterinary Handbook say: “Vitamin and mineral supplements are not necessary if you are feeding a nutritionally balanced diet. In fact, they may even be harmful. If your kitten is a poor eater and you think that supplements may be needed, discuss this with your veterinarian.”
The answer to the question in the title is not to feed your cat vitamin supplements but to feed him/her decent quality cat food which is described as complete and balanced. There is nothing more to add to the article if you are feeding commercially prepared cat food.
It would be a completely different story if you are making up your own raw cat food diet. It’s quite rare to do this and vets dislike it but if you are you will certainly have to focus on supplements, vitamins and the essential nutrients that a domestic cat requires. You can’t simply mash up some chicken and give it to your cat. It would lack in essential nutrients. You can read about providing your cat with a raw diet by clicking on this link.
You can read some more about vitamins and cats if you wish by clicking on one of the links below.