[Tilted Irish Kilt]

greeneyes :" It’s almost impossible to overwater that mix, and the vermiculite holds water and helps during the dry season."

Vermiculite (heated mica ) does hold some water , but it has only a medium rated analysis on water holding ability.
Perlite is a sterile medium , holds no water moisture, but makes soil more friable (break up) , and is reccomended for seedling developement , or in heavy soils (ie.: clay).
In both instances , it is recommended to use with peat moss, or sphagnum moss for stabile moisture for plants.

[greeneyes]

There were several reasons that Mel gave for using vermiculite instead of perlite, but the only one I can remember is that it holds water better than perlite.

And yes 1/3 of the mix is supposed to be peat moss or sphagnum moss. Peat is readily available here.