If a physician has "prescribed" certain vitamins for my autistic child, and I have the notification, can I claim said expenses.

[DoninGA]

If you are purchasing over-the-counter vitamins, those are not a deductible medical expense.

[Opus 17]

Yes, they are allowed, but only if you fill them at the pharmacy.  If you buy them over the counter, they are not allowed, even if they are prescribed.