What action (if any) should I take? Vet Malpractice?

If you can prove that your dog was over dosed by the vet, you should report the vet to the proper regulatory authorities. I would also tell the vet that he/she should be responsible for any and all treatments and meds that occur as acresult of their mistake, including another vet. If the vet would not agree, I would retain legal counsel and pursue it in court. I'll bet your vet's malpractice insurer would not let it get to court.