Vegemite is a yeast extract spread popular in Australia and New Zealand. It is very salty and has a strong, savory flavor. Cats are obligate carnivores, meaning they need to eat meat to survive. They have no need for the nutrients found in Vegemite and their taste buds are not designed to enjoy such a strong flavor.
In fact, Vegemite could be harmful to cats. The high salt content could lead to dehydration and other health problems.
If you have a cat, it's best to stick to cat-specific food and treats. They'll thank you for it!