I don't like answering my own question, but because I came across this article below and it's slightly different than the answers above, I thought I'd post it as an alternative answer. I also respect the "don't give cats supplements" or "a drop of fish oil" answers, too. I don't necessarily think this is right, but it's an option to consider.
Article.
It quotes a study that found that cat's, unlike people, don't get vitamin D from sunlight, even when shaved.
Cats get vitamin D from their diet.
Food sources of vitamin D include fatty fish, such as salmon, egg
yolks, cheese and other dairy products fortified with vitamin D.
In my opinion it's better to give your cat a portion of salmon or scrambled egg or dairy (my cats love cream cheese) than a supplement. I didn't ask this question to give my cat vitamin D as a supplement, I asked cause when my cat sees me take a vitamin D she asks for one and I wondered if maybe she knew something I didn't, but I also don't trust a cat that eats dust bunnies to know what's good for her.
It's also possible that cats that like beef and chicken cat food only might be a little low on vitamin D, unless those cans are fortified. My cat eats a fair bit of tuna cat food, so she might be fine on cat food alone.
I did ask the vet "she eats all kinds of stuff" and the vet's answer was "maybe she has a vitamin deficiency" - I'm not sure I believe the vet, as that's just one answer, and a natural instinct to chew on stuff is another, and the vet sounded more like she was proposing a possible answer than stating a fact, but it's possible.
Also from the article:
The U.S. Food and Drug Administration’s Animal and Veterinary
Department recommends vitamin D levels in foods that vary based on a
cat’s development age. If your furry friend is a kitten or pregnant
female, she should receive 750.0 International Units per kilogram, or
IU/kg. For adults, that number drops to 500.0 IU/kg.
and for toxicity
Both age levels have a maximum daily amount of 10,000.0 IU/kg.
I supplement my diet with 2,000 IU per day (doctor recommended dosage) and my kitty is between 4 and 5 kg, so surprisingly, 1 of my pills is about my kitty's daily recommendation, but that doesn't include what she already gets in her diet.
To be safe, I may give my kitty 1/2 pill, which, given that it's oil filled, would be measurably less than 1,000 IU with spillage and see if she likes it, not every day, just when she asks for one. I think that's probably in the safe zone. I still have no idea if my kitty has a vitamin D shortage (people go years with a vitamin D shortage and they don't even know it - so, it's not all that bad). But I think a half pill (less than 1,000 IU), every once in a while, should be perfectly safe and a plate of salmon or bit of dairy here and there. Dairy is one of those "should I/shouldn't I" things too, because some cats have trouble digesting it - but I'm sure that's covered in other questions.
On how much is in cat food
This article says that good quality cat food contains plenty.
Good-quality cat foods already contain great levels of Vitamin D.
Also, many of our cats’ favorite foods, such as oily fish, eggs and
cheese, are already high in Vitamin D. Giving a supplement just isn’t
necessary — and it’s dangerous to give too much of a good thing.
Your cat is unlikely to eat enough food to become poisoned with
Vitamin D. Indeed, the most common causes of toxicity are
over-supplementation or eating rodents poisoned with Vitamin D–rich
baits.
The article also indicates that higher levels of vitamin D can help sick cats heal, but whether this means cats should have more vitamin D than they typically get is unknown. More research is needed.
In May 2015, the University of Edinburgh vet school announced the
results of a study showing that seriously ill cats with high levels of
Vitamin D in their blood were more likely to be alive 1 month later
than cats with low Vitamin D levels.
and it goes on to say:
It’s easy to leap to the conclusion that if Vitamin D aids in
recovery, then giving your cat a Vitamin D supplement could improve
his health. However, it is dangerous to make this assumption because
Vitamin D is toxic in high doses.
At this time, in the (in)famous words of former U.S. Secretary of
Defense Donald Rumsfeld, “There are more unknowns than knowns.”
How or why Vitamin D benefited the sick cats is not known, which is
just one reason the experts do not advise giving your cat a
supplement.
and just for fun and further reading, another article that discusses the uncertainty, or, as they put it "controversy".
I want to point out that the first article quotes a study that cats don't make vitamin D from sun and the 2nd and 3rd article says they do - so, who to believe? Personally I believe the study, not the argument that because humans make vitamin D in sunlight, cats do to, but it still gives me pause when sources make contrary statements.
My conclusion - vitamin D supplementing is likely not a good idea without a vet's recommendation and a blood test, because for now, not enough is known. Cat food contains vitamin D, and supplementing with vitamin D rich foods like salmon or other fish or dairy or even a drop of fish oil is sufficient. There's no reason to ever give a cat an actual supplement, unless the cat doesn't like fish, egg yolk or dairy so the diet might be a little low and even then, only with a vets recommendation, blood test and in kitty portions.
Again, I never considered supplementing my cat for vitamins. That wasn't the question. I only asked, because she wants one, so I wanted to know, if I gave her one (or half of one), as an occasional treat, would it hurt her. Based on reading, a half pill (considerably less than 1,000 AU with spillage) and not very often, sounds within safety limits. I'm going to give my little crazy a half pill next time she asks because a small enough dosage sounds well within the safety range.
But wow, vitamin D and cats. To quote Donald Trump. "Who knew it could be so complicated."
