Vermiculite is already as sterile as you need it to be for incubation. Eggs do not need a sterile environment to hatch - lizards bury them in soil or termite mounds or whatever - they are exposed to bacteria, fungi, and a heap of other potential pathogens. Other reptiles likewise. There is always the risk that your babies will hatch with reduced immune responses if they are not exposed to some challenges during incubation. If eggs die during normal incubation with vermiculite, they either weren't meant to hatch, or your management parameters are incorrectly set.
Jamie