Cats don't need vegetables in their diet. Cats are obligate carnivores, not omnivores or herbivores. A cat can have a healthy diet without veggies.
But some cats do enjoy a little piece of veggie as a treat. A little canned pumpkin puree (not pie filling, just 100% pumpkin) can be added to food and has the benefit of helping with minor constipation. There are pumpkin supplements for pets that you can buy (Weruva's Pumpkin Patch Up) but it's the exact same thing as a whole 15 oz can of pumpkim puree from the grocery store at a marked up price for a little 3 oz pouch. Not all cats like pumpkin, though.
Personally for "veggies" I give my cats a pot of live cat grass. One loves to munch on the grass. Live catnip in a pot is also a good "veggie" plus you can dry the leaves and use them to stuff toys and to re-fresh old catnip toys.