Do vitamins help our memory?
There seems to be no effect of vitamins. This paper ("Preventing cognitive decline in healthy older adults." found no evidence for herbal supplements, vitamins or fatty acids improve cognitive functions.

There seems to be some evidence ("Preventing Alzheimer's disease-related gray matter atrophy by B-vitamin treatment.") that a cocktail of hig dosed vitamins including folic acid, Vitamin B6 and Vitamin B12 can slow Alzheimer's disease. But: The benefits were only seen in participants which had high blood levels of homocysteine. This amino acid is linked to a higher risk of heart disease for which the B vitamins are postive for the outcome. So this might only be helpful for people with too high homocysteine levels.

And its questionable if high doses of antioxidants (like vitamin A or E) are not only not helpful, but can cause actual damage. This is due to the fact that they catch up reactive oxygen species, which might otherwise trigger the cell to go into apoptosis and prevent it from becoming a cancer cell. Here is the paper: "Antioxidants accelerate lung cancer progression in mice.".