Its always good to add vitamins to a birds food or water. Most bird vitamins you can put in either. As far as them being bald in some spots, its USUALLY from them plucking their own feathers out. With the exception of the female who is balding on the back of the head (they cant reach that with the beak). Again, that usually means that the other bird is doing it. Birds are smart little things. Regardless of what some people say, they can become depressed and or stressed. This is usually what causes feather pulling. Sometimes its extreme boredom. Try adding some new toys of different types. Like wood and plastic. Give them new toys every 30 days or so to keep it entertaining. Add vitamins to their diet. Give them a good variety of food and see how that goes. If it still doesn't change, ask a vet or separate them for awhile. Also, try covering the cages at night.

I would ask a vet about the vitamins, the only time I have heard of cockatiels loosing feathers is over grooming or another bird "attacking" it

Hi i just bought a pair cockatiels and the Male (theo) is featherless around the neck and it looks naked around the neck but the person we bought from said that it needs vitamins it didn't pluck me out it just was like that and the female (pearl) is a little bit feather less at the back of her head is it true does it need vitamins?