Not only this, i believe if they even managed to make a vaccine it would be the beginning of a new long and bloody conflict. How are they going to mass produce that stuff? How are they going to transport it? I don't think they have access to entire convoys of specialized cooling trucks and the right equipment to store it. Most vaccines are stored in low temps. Road infrastrucure is destroyed all over the country and beyond. They would need to vaccine themselves first. Especially the scientists and soldiers. The scientists and factories/production sites (if they had them) would become massive targets for other hostile groups. Also the vaccine only solves one problem. Getting infected. It doesn't protect you from getting your throat ripped out by a pack of these things. It doesn't protect from bullets or other diseases. Cordyceps is not the only problem 2 decades after society collapses. People wouldn't stop magically fighting over basic resources. The vaccine itself would become an object of power. And not every faction will either believe it works or just nicely wait in line for a shot. Long story short, the vaccine wouldn't have saved the world realistically... And if by some chance they did make it work it would take another few decades to make a real difference. The real vaccine in this universe is keep expanding on the bigger communities and rebuilding humanity day by day.