Snape using the Vanishing Charm in 1992
Vipera Evanesca is the counterspell for Serpensortia. It causes the conjured snake to vanish. Presumably, it is related to the Vanishing Charm.
Etymology
Vipera is a genus of venomous vipers, a type of snake. Evanesca likely shares its origin with Evanesco, which means "disappear" in Latin.