Soil should either be acidic, lime rich and sandy.
I have mentioned this previously in different posts, but believe this is one of the most missed ingredients with harder-shelled eggs. The acidity of the soil directly improves chelation of calcium and the slow break-down of the shell. This has a great impact on the avialable calcium for the developing embryo.
The pH of vermiculite will vary tremendously depending upon the source of the vermiculite being mined and used in what we buy for incubation medium. It will range from 7.0 to 9.0 - so very much in the alkaline range. Way too alkaline for calcium chelation.