I bought a 50LBS bag of sanitized playground sand for my Girl to do her lady business in. I used 1/4 of it in a medium sized plant pot and 1/2 of it in a 5 gallon bucket.
I left the plant pot in her cage so if she has the urge to go in the sand its their for her and i have the bucket to move her into if she needs to do some real digging in privacy.
I tried just play sand in a 10"X6" wide by 12" deep bucket and she "tested" one of the tunnels i dug with my finger but nothing after that. She preferred the paper towels on the floor. So I now mixed the sand & soil with the live plant and she crawls around the new larger bucket but hasn't dug yet.
for my bearded dragons I use the exo-terra soil bricks (3 pack) in a large plastic tote, it works very well, however I do recommend adding a bit of sand to it.
This is what I will be using for my female panther sometime soon when I start breeding her with my male panther. Hopefully soon. She's only seen him a few times and doesn't like him whenever he sees her... She does have all the signs of being receptive though (pinkish colour on the body and eyes)
For my Kinyongia I bought a bag of organic dirt from Lowes (can't remember which one it was, but all organic) and mixed it about half and half with play sand, it holds tunnels really well, and she happily laid her eggs in it (about 10 inches down) once she decided she wanted to. Just remember that it needs to be moist. I found it easier to mix the water in as I was filling the digging bucket, so I always had the chance (except for the very end) to add more sand/soil if I over did it on the water.
So, a good amount of breeders use vermaculate to house the eggs AFTER they have been layed, but since you need to know what to use for the initial laying, I use sand mixed also with dirt that is just moist enough for you to dig a tunnel with your hand and for the tunnel to stay and not concave
I wouldn't use any soil or plants in the laying bin. Its hard for them to dig by the roots and with soil your cham will be black til its next shed most likely. All you need is moist sand, which I put in a 5 gal bucket to stick in my girls cage when its time.