Vs a Vs the breeder. It is quite hard for me to fathom why a deformity that was present from birth was not noticeable until 6mths old. Not only did the breeder miss it, her vet missed it, and pardon me but your vet missed it!
You have been let down by not only the breeder, but by the veterinary community at large!
It is stories such as this that make me very angry, which might not be of any help to you, but still there it is.
Of course this is given that the medical condition *what-ever it is* was something that would have been noticeable if not at birth by at least 8wks old.
If that defect is genetic, I think I would document the whole report, Xrays, professional opinion etc, and send a registered letter to the breeder. I would ask for full return of money back given the exigent circumstances, and if their was a failure to diagnosis such a condition on the breeders' vet part I would really push for that money. You should not have to suffer the burden of such pain! That is my opinion of course.