I played Vagrant Story and it is awesome.
I played XII and I felt let down.
However, I will admit that the world & history of XII was very interesting. Possibly more interesting than the main story. I would try kill creatures so I could look in the bestiary and find out more about them.

XII's story is not terrible, but it isn't great either. I agree with Captain that it feels like there are elements to work with, but they are not utilized. Look at the character Al-Cid (or whatever his name is). This is an interesting character. But his screen-time is very limited.

There's Rozarrio, which you never go to. Irritating. There's Vaan's brother, Penelo's brothers, their dead parents, all of which get a brief mention but aren't properly fleshed out. I mean the two of them were orphaned - they should have some emotional damage or something to make them interesting. And they should be damn angry with the Empire!

And that is the crux of another problem. There seems to be a lack of emotion throughout the game. Ashe is one of the most detached & unemotional characters ever. Basch is mechanical. Fran is cool as well, but it's done in a way that is endearing and makes her likeable. Penelo & Vaan: you're an orphan because of the empire. Hate them loudly please.

There is only Balthier as a main character. Along with Reddas, they saved the game for me. But neither character is properly fleshed out.

I don't think XII's story is bad. It's decent. But what does irritate me is that there was a lot of unrealized potential.