Vermiculite will NOT increase your cation exchange capacity. Vermiculite
is not a clay, has low surface to volume ratio and is an extremely poor
choice for increasing the fertility of your substrate. The only reason
you might want to use it is to decrease the density of your substrate.
You are paying top dollar for the Dupla product, and while I feel that
its not worth the money, there is a lot of experience backing this
product so IMHO follow their instructions!