Basically, a vet's liability regarding vaccines comes down to 3 questions:
1. Did the vet use the vaccine in accordance with standard veterinary practice? Do other vets reasonably use it that way? This does not mean the vet followed the labeling guidelines, but that he followed common practice.
2. Did he obtain informed consent? Did he tell the owner that there was a risk of adverse reactions? The owner should have been made aware of what the benefits and risks of the vaccines are and if there are options.
3. Did the vet provide undue warranty? Did he claim that the vaccine was 100% effective? Basically, no vaccine is 100% effective for all cats. For example, the calicivirus vaccine lessens the infection but does not prevent it.