one more thing, if you do give any kind of treat to administer the liquid B complex be sure to provide size appropriate grit on the side in a dish near the food
in a situation like yours, where the birds need a boost of niacin fast , many of us use this , 1 ml per day per bird, over a treat like meal worms or given directly orally - but be careful not to aspirate the birds...
you don’t have to worry about overdosing niacin because it’s water soluble. add 1TBSP NUtritional yeast to every 1 cup of feed.
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
please post pictures showing how your ducks legs are bowed