If Sulfuric Vapors is in play and I deal 3 damage with Incinerate to a target, then Healing Salve is played to prevent 3 damage, how much damage is successfully dealt?
The net damage is 1. Sulfuric Vapors adds the additional damage before damage-prevention abilities are played. A spell or ability deals damage when it is played, and any relevant effects trigger at that time. After all damage-prevention effects have resolved, any remaining damage is considered successfully dealt. At that time, effects that trigger on successfully dealt damage occur.