Admittedly I haven't... been all that far from home. I haven't been able to hear or see much what it's like in other countries. But the hunter's headquarters used to located close to where I lived before. I would say that things are more concentrated within Japan. But vampires can and do live all around the world.
[There's no law saying they have to stay in one location. It's only that their main government body is there.]
I'm sure there are hunters stationed in other countries as well, but the parent metal used to be within Japan.