I have just finished the revision of a paper describing the history of allergy development. While a temporal coincidence (introduction of vitamin D) will not provide any proof, the absence of such a coincidence (like key features of the hygiene hypothesis) is a rather strong argument against a hypothesis.
It has been repeatedly claimed that drinking unpasteurized milk will protect against allergy […], while the history of pasteurization makes such a relationship unlikely […]. Pasteur developed it in 1864, the procedure was introduced in 1889, and already by 1920 all commercial milk underwent pasteurization in the U.S. Pasteurization therefore cannot be the culprit, although avoidance of pasteurized (and additionally vitamin supplemented milk) may indeed relate to lower allergy rates.
Here is a picture form the Maison Pasteur that we visited recently in Arbois.