I know very little about vitamin B15, but this 'vitamin' is - according to the site quoted below - to be found in brewer's yeast, whole grains, seeds and brown rice - stuff most vegans are known to consume on a regular basis. According to the same site, too much B15 can cause severe fatigue, pernicious anemia (long-term very high intake), depression, headaches, skin ash, shortness of breath, nausea, and increased vitamin B12 requirements.

Who knows more about vitamin B15?