I know you must be very close to your Bearded Dragon lizard, and I know this matter is very important to you, so please forgive me if I say something disrespectful or flip.

Suppose for a moment you were to take the vet to court for veterinary malpractice or negligent lizard-sitting or whatever. You would have to prove causation, that is, that the vet did or didn't do something to cause his injuries. It is not going to be obvious to the court that his injuries were caused by the visit to the vet, especially since you say the injuries manifested themselves "a few months" after he returned home. You would need expert testimony from a qualified herpetologist, not only that it is possible for a Bearded Dragon lizard's injuries to manifest themselves months after the fact, but that these injuries, more probably than not, were caused by lizard abuse while in foster care, as it were. (Just as one must not shake a baby, one must use care in handling Bearded Dragon lizards to avoid severe chest and tailbone injuries.) The cost of retaining a herpetologist might well be prohibitive. You would also have to prove damages. In the absence of proof of willful cruelty, your damages would be limited to the fair market value of a similar reptile. The fact that all this happened "a few years ago" also suggests you may have statute-of-limitations problems. The statute of limitations for veterinary lizard malpractice is generally two years. I am sorry you are left with a listless, limbless, lethargic lizard. This may be another sad example of a wrong that cannot be remedied by our court system. The court cannot give your lizard a new tail or foot; the best the legal system can do is to award you money, inadequate as that may be. You certainly have nothing to lose by taking the vet to small claims court, but my best guess is that she would shed this lawsuit like so much skin.

I agree with Michael's eloquent answer.

Good luck.