Without testing, you can’t really go based off the symptoms alone, however, what I CAN do is suggest possibilities of what it COULD be.
Slipped Tendon/Pulled Muscle or Broken Bone — If your chickens are active and have places where they can jump off and to that is high (such as a tree branch), this can most definitely cause a loss of function of the legs or even a possible break. Most chickens will put weight on the sore leg, but will not walk very far or will limp. However, a slipped tendon or a pulled muscle usually affects one leg rather than two, but anything is possible. Same thing here— especially if your chickens can jump on and off from someplace high, they can break one or both legs from the fall. This isn’t very common though— especially with both legs, but as I’ve said, anything is possible with chickens!
Vitamin Deficiency — Since you mentioned that she’s a vault-headed breed of chicken (ones with a largish crest), I go into thinking that this could be from a vitamin deficiency, specifically vitamin B-12. Treat with vitamins or a vitamin B-12 complex for a week or 2 to see if she seemingly recovers.
Marek’s Disease — I’m aware that you mentioned that you don’t think that it could be Marek’s Disease because they are vaccinated, however, this is not true unfortunately. The vaccine for Marek’s Disease DOES NOT prevent the viral infection from affecting your birds, it just slows down the progression at which the tumors inside form, giving the birds a longer life than those without the vaccination. This is still a possibility, even though vaccinated. Only a necropsy or testing can tell you wether your birds will have Marek’s Disease or not.
Avian Leukosis Virus — Just like Marek’s Disease, Avian Leukosis Virus (ALV) is a viral infection of chickens that causes internal cancerous tumors to form, causing neurological issues, mimicking Marek’s Disease as well as other existing diseases. ALV does not have a vaccine available and infected birds can recover, but always carry the virus for life as well as passing the virus through eggs in the embryos, making any chicks that hatch, already infected with the virus. The only way to know if ALV is what is ailing your birds, is to get testing done to confirm.
Botulism — This is a serious bacterial disease of anyone and anything, in chickens and other poultry, it can cause neurological symptoms, nerve damage or even death if not treated. Most birds that are infected, will die within 2-3 days after initial infection, making it difficult to find and treat it right on the spot. This bacterial infection can come from moldy feed, dead carcasses and other objects. Aggressive antibiotics are needed for birds to fully recover from the disease.
I hope my small list and small descriptions of them, help you in any way. I still suggest to you that you get testing done as soon as possible.
