I know in humans that lack of vitamin A can lead to vision problems.
I know lack of Vitamin A can lead to many problems in parrots.
Do not feed vitamin A itself, as too much can be toxic.
Instead feed beta carotenes which are converted to vitamin A or excreted.
Beta carotenes are in carrots, red peppers, broccoli stems, sweet potato, cantaloupe. a great source is red palm oil, but this is very fatty so don't feed gobs of it.
At least it won't hurt anything to have a good diet.