You can cut the plant back, leaving just a couple of leaves on the bottom. Take the cuttings and strip off the lower leaves--one or two of them, leaving the leaves above intact. The original plant will put off more stems, often two above where you have cut. After you have stripped off one or two leaves from the bottom of each cutting, leaving the top ones, you can plant the cuttings in soil right in the terrarium, up to the live leaves. The roots will come out of the leaf nodes where your have stripped the leaves off, so the nodes will be under the soil. You can also choose to simply put them in a glass of water in the same way until you can see roots forming, then plant them later if you want-- or root them in a separate pot of soil.
It's not a measurement in inches, but just burying the leaf nodes from which you have stripped the leaves because that's where the roots will form.
This works well with most vining plants and many others. Some aren't as easy and require a bit more invention, but your Peperomia will work just fine.
Whatever you do, it's probably going to attempt to take over your vivarium anyway. I have one now in my paludarium that is invading the aquarium portion. I suspect it of attempting to eat the fish. I think it has its eyes on eventually crawling across the continent and eating New York City.
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Patty
Lost River, Idaho