I knew of two puppies with that condition. Happened around 8 weeks of age and owners were told it was common in the type of breed and many medium - large breed pups. I think your pup might have something like that. Look up Hyperflexion deformity in puppies. A friend of mine found a Great Dane breeder online who has a treatment system for this and he used her product with great results. There is also a website showing photos of this condition but I forgot where we found it when Duke had the problem. If it is hyperflexion deformity I understand you don't want to give extra vitamins.
http://www.infovets.com/free/canine/default.asp?page=F/F610.htm
http://www.infovets.com/free/canine/default.asp?page=F/F610.htm