According to ESRB, this game contains: Animated Violence
I feel a bit a uncertain on what to say to parents. Vagrant Story is no doubt a mature game if there ever was one, but not in the sense that it is particularly bloody or violent. Although there isn't anything very inappropriate for children in it, I have trouble believing many younger players would be interested in religious trappings of its plot and the complicated nuances of its combat system (although this could be true many adults as well).
I feel those who like Vagrant Story will be those who know what to expect from it. It's a dungeon-hack, pure and simple. All you do is explore, fight, and customize your weaponry. If that's not your thing, hey, you know who you are. If that is your thing, however, I cannot think of a game on PlayStation that does it better.
In terms of general appeal, Vagrant Story is somewhat a double-edged sword. On one hand its visual style is so dramatically effective its story could easily be enjoyed by anyone.
Yet on the other hand, its gameplay mechanics are so intensive it is most likely to appeal primarily to hard-core RPG gamers. The casual and even not-so-casual gamer may find themselves frustrated with the well-designed yet daunting battle/weapon system. However, those with more veracious appetites for strategy will surly find Vagrant Story to be a fat slice of gaming heaven.