I use B complex or super b complex tablets. You can crush them in food or a little water, or give a 1/2 tablet orally. They have all b vitamins including thiamin’s, riboflavin, niacin, b12, etc.
How old is the duck, what is her diet, and with what vitamins have you treated her wry neck? Is she given extra niacin, b complex, or nutritional yeast?