If you are looking for quick information on vitamin D3 this factsheet page from the NIH is pretty good. We all have known about vitamin D for our bones. Osteoporosis etc. What has been less known is vitamin D3’s impact on the immune system and other metabolic processes. Not that I am a website critic, but the page is clean and to the point.
The bottom line is to make sure you are getting enough vitamin D and maybe even boost it. You can have too much vitamin D however and should modulate your intake per trusted sources. Vitamin K2 is mentioned as a vitamin that helps with mitigating risk of harm for people taking high doses under a doctor’s care.
General Vitamin D supplementation is viewed as safe. The vitamin D Factsheet at the NIH is a good start in your research