Glad that you were able to see the vet. B complex tablets would be good to try, rather than b 12. Riboflavin is in the B complex. Mareks might still be an issue even though she was vaccinated, but I hope it is not. Let us know how she continues to get along.
Many times the treatment does not help. The important thing is to diagnose it. Here is some info about finding the tendon and moving it into the right place, and splinting it. Here is some reading about slipped tendons...
Did she ever have any problems with that leg earlier on? Slipped tendons or a leg bone deformity can be common. Varus valgus deformity or a twisted tibia can worsen with age. Sometimes a hock tendon may rupture. I hope that she improves.
Here are some examples of slings, chairs, and a nest:
https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/
And here is a video:
Sorry about your chicken. Was she vaccinated for Mareks disease? Did you witness her injuring the leg? Would it be possible to see a vet tomorrow for an xray? At her age, I would worry that it is a sign of Mareks, but it could be an injury. A chicken sling might be something that you might want...