our first flocks (soay included) werent vaccinated but mainly down to our inexperience, naivity, and a laid back vet who always said no point in a small flock of sheep -or pigs for that matter (probably as to him they werent worth much to replace)
when we restocked with pedigree sheep, we vaccinated as they were worth alot to us emotionally and financially. (and next time we get breeding pigs, they will be done too.)