Vipers are poisonous snakes that are dangerous and represent evil. Here "offspring of" means "being like." Alternate translation: "You evil poisonous snakes" or "You are evil, like poisonous snakes"
John was rebuking the people because they wanted him to baptize them, but they did not want to stop sinning. Alternate translation: "You cannot flee from God's wrath like this!" or "You cannot escape from God's wrath just by being baptized!"