There is a much disagreement on the internet about the source of vitamin D for cats

Cats get it from prey/food

Example How Much Vitamin D Should My Pet Receive?
Dogs and cats cannot synthesize vitamin D-3; in the wild they would obtain it by eating prey so it is added to all their complete commercial pet foods.

Cats get it from their fur if they get time in the sun

ExampleAutogrooming by cats

• A cat acquires the essential vitamin D through licking its fur after exposing itself to the sun.
• Self-grooming also stimulates the glands at the base of each hair. The secretions of these glands will keep the fur weatherproofed.

Example Feline Nutrtion
Cats cannot manufacture vitamin D or its precursor 7-dehydrocholesterol. Cats appear to have an extremely low dietary requirement for vitamin D provided that they have exposure to some sunlight and are otherwise well nourished.

Metabolic bone disease in lion cubs at the London Zoo in 1889: the original animal model of rickets
Full text online

Cats like UV

Two of my friends have cats which are now enjoying a UV lamp similar to the low cost one which I use in the winter.

Cat and Dog distemper

WikiPedia
Vitamin D Council Oct 2011 mentions one cat owner who used vitamin D to treat the cat distemper.
Vitamin D does prevent and perhaps treat virus infections in humans (flu, etc.) Overview Influenza and vitamin D

Zimbio July 2011

In an animal with distemper, demyelination causes the lingin of the nervous tissue to be destroyed.
Subsequently, the dog`s body attempts to fight this by using its own immune cells.
The result is encephalitis, inflammation of the brain, which leads to seizures.

Phenobarbital is thought to be a good way to help fight against and limit your pet`s seizures.
It might also be wise to supplement this with a Chinese herbal formula that will aid in healing the damage already done.
On top of that, hemp oil, vitamin D and B, kelp, supplemental magnesium, and vitamin C can all have a positive effect on this condition.

See also Vitamin D Life

• All items in category Veterinary and D 130 items
• Overview Veterinary and vitamin D