Merriam Webster's old definition contradicted other dictionaries and actual usage. Probably you heard about diphtheria and tetanus vaccines before 2020. Those are prepared from toxins. And Merriam Webster's virus definition said viruses aren't organisms. So their old vaccine definition excluded anything made from a virus. Other popular dictionaries had better definitions and didn't change them.

The CDC working definition didn't change. They removed a redundant reference to protection from a page for the public because some people claimed it meant anything under 100% effective isn't a vaccine.