It's possible for a vaccinated chicken to get the Marek's virus and become symptomatic. The vaccine only causes the vaccinated chicks to develop resistance to the virus. If a vaccinated chicken is exposed to the virus, and if it's a strain not included in the vaccine, and exposure needs not be direct, virus particles brought in on the soles of shoes or shed off clothing can infect. And that vaccinated chicken can then go on to infect other chickens. It's rare, but does occur.
Then, there is not just the Marek's virus. There is also an even more common avian virus that can cause lameness and paralysis - lymphoid leucosis. Marek's vaccine will not prevent this virus. It's also very contagious and can be brought home to your flock on soles of your shoes.
There is also a chance your hen is experiencing, not avian virus symptoms, but a vitamin deficiency. Riboflavin deficiency can cause lameness. It does not harm to try B-complex therapy to see if it can help. Get the B-100 complex as it has high amounts of all the B-vitamins that can help with symptoms such as your hen has. Give the B tablet directly into the beak daily.
How old is she, by the way? That can have bearing on what odds she has of this being Marek's or leucosis.