There's an old thread where Rich Miller posted some great tips in dissecting hull #294 "Clean Sweep" http://j30.us/ubbthreads/ubbthreads.php/topics/5934/Dewatering_Aft_Bilge_Need_Advi.html
He had some comments on the vermiculite.

His pictures & descriptions are here: http://j30.us/J30%20Rebuild/Inner%20Laminate.html

Rich seemed pretty knowledgeable on this, so you may want to contact him.