When making an attack of opportunity, does it count as a standard attack and thus benefits from feats such as Vital Strike?
A creature can't benefit from the feat Vital Strike when making an attack of opportunity
The Pathfinder Core Rulebook FAQ includes this exchange:
Can I use this with Spring Attack, or on a charge?
No. Vital Strike can only be used as part of an attack action, which is a specific kind of standard action. Spring Attack is a special kind of full-round action that includes the ability to make one melee attack, not one attack action. Charging uses similar language and can also not be used in combination with Vital Strike.
Emphasis mine. Unfortunately for the creature with the feat Vital Strike, the free attack granted by an attack of opportunity is not a standard action but not an action at all. A similar conclusion is reached in Paizo messageboard threads from 2009, 2010, 2011, 2013, 2014, and 2016.
Attacks of opportunity are not standard actions since they are used during enemy turn, and Vital Strike explicitly requires an attack (so standard) action.
This will be the case for any kind of feat or class feature that requires an attack action or a standard action.