I believe that just like the TSH tests for Thyroid hormones are manufactured to sell drugs not cure so is the Vit-D test.
I mean we know sunshine, and even on a cloudy day enough reaches us to trigger this, creates a whole array of hormones that tightly interact with each other. So now we only test for a single building block of a single one of those hormones.
Interestingly we load up on the one we can show is an immunosuppressive steroid..
I could go on but this bores me now :)