DavidHC, annoyingly the article that you linked is behind a paywall for me as well - my host institution does not have an e-subscription to Gene.

The Vitamin D Receptor (VDR) is a transcription factor. What that means is that when it binds the active form of Vitamin D (1,25-D) it moves from the cytoplasm to the nucleus where it can potentially switch on the expression of many hundreds of different genes. There is an account of this here; http://mpkb.org/home/pathogenesis/vitamind/metabolism.

This whole field is very complex - switching on or off the expression of a given gene is rarely controlled by a single transcription factor - they work in "teams". Hypothetically, switching on expression of gene A might need VDR as well as transcription factors X and Y but this could be blocked by transcription factor Z.

Many Single Nucleotide Polymorphisms (SNP) in the VDR gene have been identified. The Taq1 SNP, mentioned above, formally Rs731236, has been linked to many health conditions (https://www.snpedia.com/index.php/Rs731236).

As far I can see, to date these are just correlations. More work will be needed before we understand the mechanisms which are involved and then figure out how to intervene in a clinically helpful way.