Hi Father Steve,
In my view, the whole history of the vitamins (which is far from finished) is a beautiful example of the openness (or "disunity") of science. Contrary to earlier chemical entities, vitamins were first identified and characterized by the effects of their absence, which was a completely new paradigm. Decennies passed until they were isolated as pure substances (I had the chance of knowing the Swiss professor Paul Karrer, who contributed a lot to this area, in his old age).
The first vitamin to be discovered was found to prevent beriberi and was thus called vitamin B. Much later, this factor was found to consist of several distinct types of molecules: B1, B2.. Some discoveries later proved erroneous or redundant, thefore some numbers are now missing. Vitamin A, the second to be discovered was so named from "anti-xerophtalmic substance". Once the first two members of the "series" were established, the naming was continued according to the alphabet, again eliminating some members in the course of events.
(source: Ullmann's Encyclopedia of Industrial Chemistry, VI ed. 1999, Electronic Release, Wiley-VCH.)