It's been accepted for some time, or as long as I can remember, that marine fish can garnish a temporary immunity to ich. It's not unlike people getting the flu, then being immune to it for a period of time. I would assume a crypto vaccine in fish would work the same as vaccines in humans. Fish get the vaccine, and for a while they are immune - but eventually they will need re-vaccinated. As I mentioned before, often aquarists assume they are dealing with crypto (ich) and it's another parasite. Often I talk to people who claim to have gotten the flu vaccine and still get the flu - when in reality it's more probable they had bronchitis or a sinus infection. Even if we had an ich vaccine, proven to work, that could be bought and easily administered to fish - I can't imagine it would take the place of a good QT measure.