I have tried this last night on v1.4.12 in endless mode. I plague killed all the humans and then tried to have them research the vaccine. I had plenty of venom lying around the map and the Medication and Adv. Meds discoveries. Took a few tries (in endless mode you can spawn as many new humans as you want) to get them to discover the vaccine. My guess is that you need a really balanced economy/food production chain to be able to develop the vaccine. The problem is that even if you have all the conditions, the humans will still be trying to do higher priority tasks like resource gathering etc... in summary the game code probably does not assign a high priority wight to the vaccine research. But if a plague is in progress maybe this should be given the highest priority...