Shaik, you will have to roll the vets when you already have 100 vets. vet packs won't work above 100 vets :) The chance will of course be lower as you gain more and more vets. So, you should do it when there is a vet conversation event

http://www.nf-guides.com/sailors.htm#b2b

eh If you got vet packs use it. Say you already have 90 vets in 1 sailor use 9 1+ vet and 1 10+ vet.