Being vegan, I do blood test every once in a while (twice a year usually).
I take B12 supplements and I have my D drop this year for some reason, probably cuz I was depressed few months in summer (July-October) didn't get out.

B12 is super important for our body and being below the normal 'low' range is dangerous, and as bipolar it's super important to keep our body healthy
Drink supplements! :P

Not sure about vitamin D's role, gonna read some now...