Painful Blow (Combat)Source Arcane Anthology pg. 31
You have learned how to land blows so painful it is difficult for struck spellcasters to concentrate.
Prerequisites: Vital Strike, base attack bonus +6, Knowledge (arcana) 3 ranks.
Benefit: When you use Vital Strike (or Improved Vital Strike or Greater Vital Strike), the creature you hit with the attack must treat the damage dealt as continuous damage until the beginning of your next turn. This forces the target to attempt a concentration check if it casts a spell (Pathfinder RPG Core Rulebook 206). You can use this benefit twice per day, or three times per day if your base attack bonus is +12 or higher.