Here's why I, as a vegan, don't feel the need to take any supplements (apart from ground flaxseeds) - it appears that B12 is produced naturally when you fast:
http://www.proteinpower.com/drmike/intermittent-fasting/inflammation-and-intermittent-fasting/
http://www.proteinpower.com/drmike/intermittent-fasting/inflammation-and-intermittent-fasting/