Vets are businesses too, and there are good ones and bad ones. So what's the best way to handle a complaint? According to an article on Petplace, the first step is talk to your vet.
Having a conversation, without totally losing your temper, is the best place to start. Maybe you just want an apology or your vet bill comped. Decide before going in what you want to happen in order to rectify the situation.
If you're still not satisfied, the next step could be to file a formal complaint with your state's Board of Veterinary Medicine. They will investigate and check into the validity of your complaint.
You can also contact the local Better Business Bureau, or even contact a lawyer who is versed with pet medical issues.