
I have never heard of only giving one vac to a pup, it is thought that the dams antibodies last till 6-8 weeks this may vary depending on the age that they stopped getting milk from the dam so I give vacs at 9 and 12 weeks, some countries give 3 or 4 sets to be sure they take and not be destroyed by any maternal antibodies.
One breeder of my breed lost a pup at 6 months so now titre tests before starting the vacs when the levels have dropped, she has found quite a variation from pup to pup and litter to litter.