The material below is Open Game Content from a 3PP Source
Improved Vital Strike (Combat)
You can make a single attack that deals a large amount of damage.
Prerequisites: Vital Strike, base attack bonus +11.
Benefit: When you use the attack action, you can make one attack at your highest base attack bonus that deals additional damage. Roll the damage dice for the attack three times and add the results together, but do not multiply damage bonuses from Strength, weapon abilities (such as flaming), or precision-based damage (such as sneak attack). This bonus damage is not multiplied on a critical hit (although other damage bonuses are multiplied normally).