Vitamin D and Performance
Vitamin D Primer
Vitamin D is one of those things that helps with general health and longevity, but until recently, it was not considered a performance enhancer. Here is a little bit of a primer and the ergogenic side of Vitamin D.
Vitamin D is an essential nutrient obtained both from the diet, supplements, and can be made by the body with an interaction of UVB light and the skin. The D2 version is found from plants such as mushrooms and the D3 version is from animal foods such as beef liver and egg yolks. The animal version of D3 is considered more biologically active.  Blood tests are available to test Vitamin D and measure a form in the blood called 25(OH)D which hangs around (half-life) in your body for 15 days.  It is estimated that 88% of the world population has inadequate Vitamin D levels. 
It seems that lots of tissues require Vitamin D. When scientists started looking for tissues that had the Vitamin D receptor, a surprising find was that human muscle tissue, cardiac muscle and vascular tissue had the receptor. Research into performance began.
Vitamin D and VO2max
Some studies looked at Vitamin D levels in athletes and then measured maximal oxygen consumption (VO2max). The findings here were rather mixed. An interesting finding was in an intervention study when researchers gave 14 athletes(rowers) a 6000iu dose of Vitamin D3 daily for 8 weeks and their blood score raised to 120ng/ml and their VO2 improved 12%. The researchers theorized that Vitamin D may affect the binding of oxygen to hemoglobin. 
Vitamin D and Strength/Power
A meta-review study found significant improvements in upper and lower body limb strength and found improvements in sprint times and vertical jump while no improvements were realized in the placebo group. 
In a group of 22 Judoka athletes, researchers gave a bolus dose of 150,000iu of Vitamin D3 and a 13% strength increase happened in 8 days. No improvements were noted in the placebo group. 
Researchers hypothesize that vitamin D helps to sensitize an increase in calcium binding to increase muscular contraction. 
Vitamin D in Muscle Recovery and Anabolism
Vitamin D has been shown to down regulate myostatin which limits muscle protein synthesis. This allows more muscle to be formed by the activation of the protein kinase signaling pathway (MAPK). Low vitamin D also changes Type 2 muscle fibers that are reversed with adequate levels of Vitamin D. 
Vitamin D and co-nutrients
Vitamin D interacts with a few different nutrients.
-Vitamin K2 helps to coregulate calcium with Vitamin D
-Magnesium assists with many steps of Vitamin D metabolism 
-Vitamin D may assist with Iron in the building of hemoglobin 
-Vitamin A assists with bone formation and may help Vitamin D dock with the Vitamin D receptor. 
How to take
It is recommended to have a blood value minimum of 32ng/ml. Most studies have not exceeded 40ng/ml in serum blood values. The current RDA dose for most populations ranges between 600iu and 800iu.
Caution is suggested at higher doses but many suggest 4000-5000iu daily. Ultimately, the dosing should accommodate a target blood serum score.
In one study, researchers found that 1000iu of Vitamin D3 raised blood serum values an average of 5ng/ml. These researchers found that blood values had to reach 50ng/ml before Vitamin D could be stored in muscle and fat tissue. 
 Office of Dietary Supplements - Vitamin D. (n.d.). Retrieved from https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
 Chiang, C., Ismaeel, A., Griffis, R. B., & Weems, S. (2016). Effects of Vitamin D Supplementation on Muscle Strength in Athletes A Systematic Review. Journal of Strength and Conditioning Research,1. doi:10.1519/jsc.0000000000001518 Retrieved from: https://insights-ovid-com.uws.idm.oclc.org/pubmed?pmid=27379960
 Dahlquist, D. T., Dieter, B. P., & Koehle, M. S. (2015). Plausible ergogenic effects of vitamin D on athletic performance and recovery. Journal of the International Society of Sports Nutrition, 12, 33. doi:10.1186/s12970-015-0093-8 Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4539891/
 Wyon, M. A., Wolman, R., Nevill, A. M., Cloak, R., Metsios, G. S., Gould, D., . . . Koutedakis, Y. (2016). Acute Effects of Vitamin D3 Supplementation on Muscle Strength in Judoka Athletes. Clinical Journal of Sport Medicine,26(4), 279-284. doi:10.1097/jsm.0000000000000264 Retrieved from: https://insights-ovid-com.uws.idm.oclc.org/pubmed?pmid=26535872
 Malczewska-Lenczowska, J., Sitkowski, D., Surała, O., Orysiak, J., Szczepańska, B., & Witek, K. (2018). The Association between Iron and Vitamin D Status in Female Elite Athletes. Nutrients, 10(2), 167. doi:10.3390/nu10020167 Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5852743/
 Deng, X., Song, Y., Manson, J. E., Signorello, L. B., Zhang, S. M., Shrubsole, M. J., … Dai, Q. (2013). Magnesium, vitamin D status and mortality: results from US National Health and Nutrition Examination Survey (NHANES) 2001 to 2006 and NHANES III. BMC medicine, 11, 187. doi:10.1186/1741-7015-11-187 Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3765911/
 Bosetti, M., Sabbatini, M., Calarco, A., Borrone, A., Peluso, G., & Cannas, M. (2015). Effect of retinoic acid and vitamin D3 on osteoblast differentiation and activity in aging. Journal of Bone and Mineral Metabolism,34(1), 65-78. doi:10.1007/s00774-014-0642-2 Retrieved from: https://link-springer-com.uws.idm.oclc.org/article/10.1007%2Fs00774-014-0642-2