Diminished capacity is certainly possible. It has happened to me. I've had a few occasions where illness, drugs or both impaired me severely enough to scare me. (Scarier still was two occasions where I retained my faculties but had personality changes into someone unrecognizable as me, and I was aware of it but couldn't start being me any more than I can stop being me usually.)

My mother had a stroke and for weeks afterward was under the impression that I was a nurse who she disliked.

Our minds are very complex systems of chemical reactions. They work most of the time. There is weird, marvelous, scary stuff up there, and we barely understand any of it.

I'm a doctor. I'm not aware of any classic pathology that involves cerebral vasculature and vitamin B. There are some cerebral vasculitides that might initially be diagnosed as vitamin B deficiency, and vice versa.

Quora user Lazlo B Tamas would be a good person to ask, though coincidentally he has already gave his answer to the question itself and didn't address this answer.

Blood and vitamin B are two things your brain needs to function (blood for nutrients, and vitamin B for both nutrients and neurotransmitter production - acetylcholine, dopamine, gaba, serotonin, histamine), so I suppose such an effect is possible, although a lot of it could also be credited to the fear of dying.