What else are you doing Rawman? Are you working out and sweating a lot? Are you doing cleansing enemas, etc?
This can happen to me if I do too much cleansing, herbs, sweating, etc. I have to replace electrolytes.
Even if I take a b12 pill, it can race through my potassium, and I need to replace it. My story is extreme though as I share this mineral metabolism problem with all other Chronic-Fatigue-Syndrome sufferers. This corrects over time for me as I am on raw, but there is a transition period.
A lot of other factors can come into play when approaching a vegan diet where you are preparing your foods and cutting out a lot of processed, enriched foods. Most of them have added vitamins that your body is used to having, so you may have to transition a time with some multi vitamins until you can be sure you are getting enough from the foods, and digesting enough.
It takes time usually for the body to become accustomed to all the fiber, and your microbiome will absolutely change in every way as well on a plant based diet and this will affect your vitamin assimilation for the better.