Hi,
bad news about your birds. I also think that you were just unlucky. As a particularly rare breed its possible that limited breeding stock has resulted in an inherited defect so if you buy some more you could try another breeder. If the birds were in good condition over the winter, well fed and well cared for then the winter shouldnt be the cause. If they free range then its possible that they could have picked up something that they shouldnt.
I breed Croad Langshans which are also very rare and like the vorwerk, suffered a serious decline after the second world war. I have also lost birds in similar circumstances with mystery illnesses which were probably suffering the same weakness.
My suggestion would be to buy in some fertilised eggs from a couple of different breeders and try again. Chicks hatched early in the year have more vigor so may be more robust than the last ones.
Buffy
I