Actually, oral vitamin D supplementation is quite a contested topic becuase the evidence is mixed that it has any benefit in treating vitamin D deficiency. But as to the general point that supplements can treat deficiency; sure. The vitamin C industry is worth millions of dollars in Australia, and yet there are virtually zero cases of scurvy. So quite clearly the vast majority of sales are not to treat deficiency but as a ‘lifestyle’ improvement measure. And for that, supplements do not work.