Has she been laying? My first thought, due to my experience, is egg yolk peritonitis. It's a systemic infection caused by egg yolk getting loose in the body cavity, causing infection in the organs. It is often fatal (when not caught soon enough), but I know specifically of at least one case where the duck person got the duck to a vet (it was quite a saga, as she had to contact vets outside her area and arrange for them to consult with her local vet). Baytril was prescribed, and the duck needed to be tube fed. However, she recovered.
Since the duck has been losing weight you may need to tube feed her. I know there is also syringe feeding. I have not had to to either, some think the tube is the way to go.
Now, that's not a sure diagnosis. I would get a stool sample to a vet if possible. They can check for parasites. If that's the issue, then you can give the appropriate anti-parasitic medicine. I would not just give her anti-parasitic medicine without being sure, though, especially since she's already run down.
The vitamins are good! But I think with the weight loss and loss of appetite, she's up against something serious.
@casportpony has published a thread about tube feeding
https://www.backyardchickens.com/t/805728/go-team-tube-feeding
And Majestic Waterfowl posted a video on Youtube demonstrating syringe feeding. I could not find it just now - will send this on, and add if and when I find it.
Remember the passageway to the tummy is on the right side of the back of the bill.
