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Veganism was invented in the 1940s by Donald Watson, founder of the Vegan Society (founded 1944). The number of vegans was marginal for decades, and I have never seen any sources on whether they used to take B12 supplements (B12 being the only vitamin strictly unavailable in a plant-based diet). However, some hints can be taken from the history of B12 isolation:

B12 isolation from animal livers was done as early as the 1920s, however it wasn't before the late 1950s that an isolation procedure from bacterial cultures was developed. Therefore, for a bit longer than a decade (i.e. from 1944 until the late 1950s) veganism existed, but not B12 from bacterial cultures. As the body stores only about 3-5 years of B12 before developing a deficiency, and as there are no sources that document any deficiency epidemic among vegans during the 1940s and 50s, we can probably assume that the few vegans that existed got their B12 from foodstuffs contaminated with animal feces.