Without vitamin D level optimization we will suffer from a wide range of health issues even if we don’t have a directly visible bone disease. Osteoporosis starts long before and lasts for a long time until the visible effects can be diagnosed.
There are only two vitamins that are not produced by plants. One is vitamin D that we produce ourselves during sunlight exposure and the other would-be vitamin B12. If you are a vegan it is only these two vitamins that you will have to supplement. Vitamin D deficiency is correlated with an increase in mortality rate and on top of that it is correlated with diseases that affect the quality of life.
Vitamin D is not a vitamin. It is a prohormone, a steroid with a hormone-like activity that regulates about 3% of the human genome from calcium metabolism, muscular function, immune system regulation, and many functions that are essential for life.