Vital Strike is source of many misunderstandings. It should be worded more clearly (or maybe it is too clear for most of the gamers to understand:/), but here we go again and I ll do my best to explain why it isnt going to work by the rules.
Vital strike requires you to take an attack action. Attack action is kind of standard action (most typically single melee attack or ranged attack).
Attack of Opportunity is not standard action and thus, you cant use Vital Trike with it.
Understood, thanks guys. I think a clearer way to describe it in the book would have been "When you take a standard to attack", rather than "When you use the attack action". The unclear part to me was: do opportunity attacks could as attack actions, even if they are free action attacks? Now I know, and knowing is half the battle ;)
Anyway, thanks for clarifying!