Can anyone explain this vaccine to me?

I understand the theory behind vaccines working. I understand the other side's opinion on them as well. However, I recently found out that the Hib disease only sometimes confers immunity, sometimes it does not. This makes me wonder. If having the full blown disease won't give you immunity (which is the case with every other disease there is a vaccine for), how can a vaccine with a much milder immune attack provide immunity? Can anyone provide me with a theory?