Cade lambs are notorious for dying.
A competent immune system is a prerequisite for a vaccine to be fully effective so if they hadn't had colostrum or something else caused them to not have a great start in life then maybe the vaccine was less effective.
I mean - you've done EVERYTHING on the preventables list haven't you - worming, vaccinating , feeding etc.
Very very unfortunate. I think with 60% PMs and poo samples definitely the order of the day.
So sorry for all your losses.