So vitamin D is pretty much my favourite thing in the world to talk about right now. Seriously.
Without going into a full thesis on the reasons you need it (and probably a lot more of it than you're getting) if you work inside, train inside, rehearse inside or live anywhere above 40 degrees north of the equator (that's most of Europe and about half of the USA) you're likely to have insufficient levels of it. A huge number of people, even those living in countries with long hours of sunlight have been shown to have insufficient, if not deficient levels. You're more likely to be at risk if you have a low calorie intake and low levels of body fat, so it won't come as a surprise that 70% of dancers tested have been shown to have below acceptable levels.
If might not seem that important but it's a major consideration if you want to perform at your best. Vitamin D deficiency has been shown to negatively effect muscular strength, power and increase likelihood of injury (in no small part due to it decreasing your muscular strength). It plays a major role in calcium absorption, and so deficiency leads to compromised bone strength meaning you're more likely to fracture a bone, and once you've fractured it, you're going to have to wait longer for it to heal. If that wasn't enough, the vitamin D hormone modifies almost all cell and tissue types in the body; deficiency contributes to decreased immunity, autoimmune conditions, various types of cancer, diabetes, cardiovascular disease, Parkinson's, multiple sclerosis, arthritis... you need your vitamin D kids.
I spent 6 months earlier this year researching the effect of supplementing ballet dancers with vitamin D. At the end of the intervention study, all the dancers who had taken vitamin D supplementation showed significant gains in lower body strength levels, their jumps were higher, they performed better and on top of that they sustained substantially less injuries over the season than the dancers who did not address their deficiencies.
It's winter, it's dark, it's freaking cold out, I was training in a gym with no heating where the temperature was -3 today, it's pretty obvious I'm not getting enough sunlight and I'm not getting enough vitamin D. It's so easy to address this. I'm not suggesting blind supplementation of anything, if you want to be sure of your base levels you can get a blood test that will tell you and you can make your own decisions based on that, however common sense dictates if you're not seeing the sun your levels are going to be pretty low. It's a serious consideration if you're working and training over the winter (or just live in a cold, dark country) and it can affect your general health over and above your performance. And unless you think jump height is more important than preventing skin cancer, I wouldn't take out shares in your local tanning salon, a supplement a day will do.