Vermiculate is not a good substrate for any small lizard. I know of crested gecko babies that have died by ingesting vermiculate after being hatched on it. Though used as a incubation medium is common place, it is very important to remove the hatchling as soon as possible to avoid accidental injestion of vermiculate.
vermiculate is non digestable and swells up when wet. Thus, when ingested it swells up in the stomach or intestines and can cause impactions. My suggestion is remove the vermiculate from your leo's cage. Keep it on papertowel for now. It will also be a good ideal to get an x-ray done to see if there is any vermiculite in it's intestines causing problems.
However, impactions are not the only reason why your leopard is not as hungry or active. Too cool temperatures certainly is one cause. A human heating pad placed underneath the tank set on low and on a timer (human heating pads are not meant to be on constantly, but a 6hr on, 6hr off cycle throughout the day works great for me). Ceramic heat emmitters or blue/red night heat bulbs can be used as well to keep the temperature up at night. Regular household bulbs can be used during the day for heat though leopards don't bask being nocturnal.
Other reasons your leopard isn't show as much appetite could be parasites. Taking a stool sample to your vet for a fecal is a good ideal to easily rule out parasites. (take a fresh sample, or, if not sure your leo will be cooperative the day before the vet, you can store a stool in the fridge in a small tightly sealed container or ziplock bag.)