I swore I would not give my daughter Vitamin D.

If you broke your oath, you must offer an expiation for an oath (kaffarat yamin): feeding ten poor persons, or clothing them, or emancipating a slave. If you cannot find any of these, then fasting for three days. If you see that breaking the oath is better, then it is advisable to break it and offer expiation for the oath; for the Messenger of Allah, peace and blessings be upon him, said: "Whoever takes an oath and then sees something better than it, let him do what is better and offer expiation for his oath."