I'm not criticizing the vet to be mean, I'm just pointing out a concerning lack of expertise that means other opinions and resources will need to be sought out in whatever way possible. Sure, it's understandable that the vet may not have this experience. But the fact that it's understandable doesn't help the bird.
Considering that you had another bird in your household die of intestinal parasites, I think it's even more important to rule out giardia. A bird can carry giardia for a long time before developing symptoms. Sometimes feather mutilation is the first or only symptom.