I have some experience with the canine 'snake vaccine'.
My black Lab "Gunner" was bitten on the face one night in our back yard. We rushed him to the Vet, and when told he had the vaccine on board, he said "Great, all we have to worry about is infection!"
The vet kept him overnight, he was sleepy the next day, and in 48 hours was 100%
It does not "proof" your dog against a rattler, but it does give you more time to get your pup to the vet.