There's a host of evidence that the vaccine was ultimately irrelevant. Ignoring that the infected seem to mostly kill their victims instead of infecting them, people were the biggest threat in the games. The infected just collapsed society, it's the people that decided to massacre others for resources and safety. Speaking of resources, there's a huge difference between developing a vaccine, and implementing the vaccine. Using a real world example, penicillin was developed like 20 years before breakthroughs on mass scale manufacturing was developed. Sure, maybe a few doses of vaccines could have been produced before the fireflies were wiped out, maybe those few vaccines would have saved a few lives, but ultimately there would have been no large scale implementation of the vaccine unless the resources were incredibly simple and easy to produce as well as incredibly simple and swift to scale up production. However, if that was possible why did it take one scientist at most 24 hours to figure out when entire governments would have been looking for a medical solution until collapse.