You're the type of guy to pour salt in the wound, kick people while they're down, and take advantage of the misfortunes of your enemies. 

When you cast a spell that successfully deals damage against a creature within 60 ft that can see and hear you, you may cast vicious mockery on them as well, without needing to make an additional action. Once you have used this feat, you may not use it against the same creature.

You may not use this with other cantrips, especially vicious mockery, as "adding insult to insult" is a completely different feat.