I'm sorry that your question wasn't answered in a timely manner. Please let me know if you still need help. May I have an update on Vixen, please?
That's quite a problem list:
polydipsia - renal disease, hepatic disease, heavy metal toxicosis, dehydration
dyspnea (difficulty breathing) - significant respiratory compromise due to air sacclitis, pneumonia, paralysis of the respiratory muscles
loss of visual acuity - primary ocular disease or diseases of the optic tracts in her brain
lameness/not walking - injury, infected joints, paralysis
Marek's disease (herpesvirus) should be an important consideration. Regretfully, these hens are lost. There is no treatment for Marek's.