Has it been used to incubate eggs? If so I would trash it. Used vermiculite holds mold and bacteria. Or is it still in the bag that was opened? In that case I would definitely use it as long as it was stored in a dry place.

I don't believe used vermiculite could be sterilized for re-use. I would imagine boiling it would only break it down. And you wouldn't want to treat it with a chemical because that could have ill effects on eggs.