He can parry by hitting the enemy at the same time the enemy is attacking, just like Dante. It's not a dedicated parry button but it's there, just need to time it right (the system is very lenient, so you should be fine).
Aion is just like all other WoW clones, I mean, the Mage in Aion has a Fireball spell and can make Portals. WoW ANYONE??? - Aion Boards
That is a boss thing that functions in a similar way to damage scaling in fighting games. It basically prevents the player from using an infinite string or deal a large amount of damage in that time.
Royal guard is a dante thing, so vergil needs something with a similar function but focuses on his style and aggression, and has other applications. But hey! the series is probably dead and such a fine mechanics can only exist in DMC, since DmC is more about dumping styles and technical aspects of the original series.