Ok, Lets see not sure which person still has a duck issue but lets try this:
First: Don't put oyster shells down unless you have chickens, if you do, then keep the oyster shells in a spot where the chickens can get it on NOT so much for ducks. Ducks only need 4% OR less of calcuim so check to see what is the MAX percentage of calcuim is in your feed.
Second: NO need to give scratch, this is Nothing in scratch that is good for a chicken or a DUCK, scratch should be considered a treat. However, if you insist on giving them scratch one a week during the winter months would be recommended. This will help them gain some fat and the fat should get them threw the winter, but again, Its NOT necessary.
Third: check the protein count in your feed, anything around 15% to 20% is good. you don't want to go to high because Ducks do dig a lot for Bugs, Worms etc and there's a LOT of protein in these insects or ready.
Four" Polyvisol, Chickens do GREAT with this stuff, not so much with ducks. My favorite is Poultry Drench, I have seen great results using this Versus polyvisol WITHOUT IRON for ducks. Now Polyvisol without iron is GREAT for chickens. I have seen better results on chicken versus giving a chicken poultry drench.
Now you also want to keep an eye on there poop, poop can actually tell you a lot, one if they have worms, two if they are intaking to much iron, IRON btw can harm or even cause death if giving to much. and it can tell you if the duck has eating something that it shouldn't have.
With leg issues it's hard to narrow down one problem to resolve, first the Leg should be Well examined. Analysising your flock is a good way to start, do you have to many boys and not enough females. Boys get excited when one is mating with a female and they ALLL want to join in well this can cause leg issues with the female.
Two, look at the joints are they swelled, look at the bottom of the foot, are there cuts, swelling, scabs (bubblefoot) etc.
So before jumping the gun and getting vitamins First analysis the situation. if your unable to see anything Then send some pics.. Hold the duck up with the legs dangling then send some pics of the bottom and top of the feet. With some of the members on here, we might be able to see things that you can't. And resolve the problem quicker.. We look for certain things and can PIN point the situation, not all the time but I would say We have a pretty good eye on what we are looking for. Don't you agree @Miss Lydia