In Fiction land, antidotes and vaccines work almost instantly. The fever goes down, color returns, heartbeat and "life signs" stabilize, the characters open their eyes, etc. Furthermore, to build dramatic tension, the poisoned character is usually given a very precise... um... deadline to take the antidote, and only manages to get the antidote right before death. Long story short, a vaccine wouldn't be very effective once a character's symptoms develop, let alone in the 11th hour right before said character is about to die from the disease as this trope describes.