We use Perlite rather than Vermiculite, we moisten the Perlite until it can be just formed then place it in a plastic tub with one single small air hole.
The Perlite will feel dry to touch but if you look underneath the container you can see some moisture.
We found these websites helpful.
Leopard Gecko Breeding
Albey's How To Incubate Leopard Gecko Eggs
Hope this is helpful.