Just because she had the vaccine does not mean she is safe from the disease. Seriously, I advise against getting the vaccine. I would much rather have a bird exhibit the symptoms of a disease then show no signs at all because of a vaccine. I'm sorry but I don't know of any vet that will euthanize a chicken. You can try calling the Frederick Animal Health Lab (301-600-1548). They will also do a autopsy for free to determine what is wrong was wrong with her.