Can Vital Strike be used with attacks of opportunity? The rules are not entirely clear to me on this point, so I'm hoping for some feedback from those who write the rules... although all opinions are welcome. Also, what about with Combat Reflexes? What about under the influence of Haste spells?
This is what we know, per RAW:
Vital Strike (Combat)
You make a single attack that deals significantly more damage than normal.
Prerequisites: Base attack bonus +6.
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 twice 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).
Attacks of Opportunity
Sometimes a combatant in a melee lets her guard down or takes a reckless action. In this case, combatants near her can take advantage of her lapse in defense to attack her for free. These free attacks are called attacks of opportunity.
Making an Attack of Opportunity: An attack of opportunity is a single melee attack, and most characters can only make one per round. You don't have to make an attack of opportunity if you don't want to. You make your attack of opportunity at your normal attack bonus, even if you've already attacked in the round.
An attack of opportunity “interrupts” the normal flow of actions in the round. If an attack of opportunity is provoked, immediately resolve the attack of opportunity, then continue with the next character's turn (or complete the current turn, if the attack of opportunity was provoked in the midst of a character's turn).
Combat Reflexes and Additional Attacks of Opportunity: If you have the Combat Reflexes feat, you can add your Dexterity modifier to the number of attacks of opportunity you can make in a round. This feat does not let you make more than one attack for a given opportunity, but if the same opponent provokes two attacks of opportunity from you, you could make two separate attacks of opportunity (since each one represents a different opportunity). Moving out of more than one square threatened by the same opponent in the same round doesn't count as more than one opportunity for that opponent. All these attacks are at your full normal attack bonus.