I would guess that it was something else. They can rarely recover from Mareks only to later die of tumors, but in most cases they don’t recover. It is good that she is doing somewhat better. I hope it continues. Continue to update us on her behavior.
With a chicken like yours, I would give 400 IU of human vitamin E and 1/2 tablet daily of human vitamin B complex or the super B complex. Generics or store brands from Walmart or Walgreens or similar may be less.
I had vaccinated birds and unvaccinated birds together for many years. The vaccinated birds did not shed the virus to the others. I never saw a case in my flock. In the future, I would not get vaccinated birds.
Do you have an up to date picture of her? Can she move either leg if you touch it, and are her toes curled under on either foot? Has she laid eggs recently? Did the vet check for a stuck egg? It is possible for a vaccinated chicken to get Mareks, but I hope the vet’s diagnosis is wrong. Do you...