Buffy the Vampire Slayer

Okay, flaw in either scriptwriting, or in logic of the universe.  The TV show Buffy the Vampire Slayer.  The concept behind the show was that there was only one slayer born to every generation.  Once that one slayer died another would take over.  This means though that there is only one slayer active at a time.  Furthermore, only one slayer active at a time and only in one place on earth at a time.  So this would mean that if the Slayer is in California there would not be any vampires in, say... Portugal, or Hungary, or Iceland, or Venezuela or even Arizona?  I highly doubt this, but I never saw Buffy traveling out of California to hunt.  What gives?  So much for the laws of mysticism giving a crap about humanity.