The association is established; acting on it is not advised
Genotype explains part of why two people in the same sunlight end up with different blood levels. It does not tell either of them what to take.
A genome-wide study of 33,996 people found common variants that affect circulating 25-hydroxyvitamin D, the strongest of them in GC, the gene for the vitamin D binding protein. The association is solid and it has been replicated.
What it is not is a dosing instruction. Vitamin D status is measured directly by a blood test that is cheap and widely available, and a measurement beats a prediction every time. No guideline sets a dose from a genotype.
The useful thing the genetics does say is that low levels are not always a behaviour: some of the difference between two people is in the binding protein, not in how much time either of them spent outside.
Nothing on this page recommends taking anything. If a result here matters to you, take it to a doctor or a pharmacist rather than to a shop.