Sorry you didn't get any responses before now. For some reason, this didn't come up in my "unread posts" feed until now.
The official line would probably be that they need their annual booster anyway or will have to go back to two jags to reinstate immunity next year, but in practise I personally think it doesn't hurt them to skip a year once in a while once they're on the system, and yes they won't have that additional boost to the antigens in the colostrum but they won't have no antigens, so the lambs will probably be fine.
The advice about vaccinating lambs is to not do it before 3 weeks old if they're from vaccinated mothers, as the vaccine will interfere with the immunity derived from the maternal antigens.