The basic idea I had was that vampires, as a community, would be severely limited in their ability to communicate with anyone who wasn’t essentially living in the same city - before phones and email became an easily available and commonplace thing, the only ways to pass information (except super short stuff through a telegraph, which has obvious limits) would either be through written letters or in person. Writing letters has a complication, in that if you don’t know who you’re writing to, it would be a bit of a problem to hand a mail carrier an envelope addressed to “any and all vampires” and say “take this to Paris.” Essentially, torches and pitchforks become a distinct possibility in superstitious times and places, and the likelihood of actually making contact with someone useful is unlikely. 

In person would work a lot better, except that, as discussed several times in the fic, travel becomes distinctly dangerous when sunlight = poof. In general, to travel safely requires a lot of time and money, as well as the assistance of trusted non-vampires. (The Viktor method of “I’ve got my tarp, let’s walk to Paris is not recommended.) So, again, going there and back is going to generally be a lot more work and probability of dying than it’s really worth, for long distances. 

Now comes the part of why Japan, in particular, has such an isolated community compared to continental Europe, and also where you can clearly see my biology background come into play. 

Migration, in the case of vampires, is both more and less complicated than actual animals. However, we can say that people, human and nonhuman, will get places. Reasons to even attempt such a dangerous relocation might include violent superstition, overcrowding, politically powerful enemies, extreme curiosity, etc. 

First off, the period of Japanese isolationism (1641-1853) would have severely reduced the viable opportunities for migration, which would have predominantly meant stowing away on a ship. So, we can safely say that while a few vampires would have successfully made it, most of them wouldn’t have survived the voyage and even fewer would have been able to survive once they got there, without fundamental resources like shelter and community. 

After the isolation era was over, it would have been easier - but not much. As technology advances, so does security, resulting in a success rate that might have been a bit better but wouldn’t be too great. 

Next up, in my world, Japan has its own native supernatural communities based on Japanese folklore etc. Some of them would have been welcoming to the newly arrived vampires and helped them out, but others would have not taken kindly to a horde of bloodsucking undead monsters showing up without invitation. 

Finally, there’s the population bottleneck. Because the founding population was very small, without a regular influx of successful newcomers, there is a limited amount that these founders could know about vampirism (they’re only individuals, after all) and the amount they could successfully pass on to anyone they turned would be even lower. As such, subsequent generations are in a position of knowing basically jack shit about the details, except for obvious stuff like “drink blood” and “don’t go out in sunlight.” Some of what Yuuri is researching in the story - the example that comes to mind is the overall success rate of turning someone into a vampire, if certain things like how they died or how old the sire is affect whether it works or not , or why/whether they have to be buried before waking up and what the limits are on that - require collecting a frankly staggering amount of anecdotal evidence because it’s not quite possible to follow the scientific method here without, uh, killing people. Essentially, this is a general loss of culture and information, which is going to lead to vampires dying earlier and more frequently because so many dangers are unknown. 

Enter Minako and Yuuri’s family. My backstory for Minako is that she is a tree spirit, and her tree is on what is now the Katsuki family property. The Katsukis are pretty chill, and for generations they’ve been super cool with all this - she’s basically a member of the family, like “oh hey that’s Aunt Minako” and have helped protect her tree for many years so she has more freedom to move around and doesn’t have to guard it all the time. Minako also has a history of taking in supernatural people who are in need, giving them a place to stay for a while (the onsen, often), and helping them get back on their feet and to the next safehouse. She has a huge network of friends who do the same thing all over the country. As a result of this, she would, if not have directly helped out vampires due to their scarcity, know people who had tried and failed and lost friends due to this lack of resources. 

Enter Yuuri. Yuuri grew up being fairly familiar with the idea of magic and magical beings, due to his family’s relationship with Minako and her guests. His backstory, which I didn’t really get a good chance to explore in Heart, is that he became a professional ballet dancer and retired from it in a way that parallels the Sochi GPF - his dog died before an important audition for a leading role that shouldn’t have even been a question, and he decided to quit dancing due to how terribly it went. However, Viktor wasn’t there to drag him back. After this, as we see in the show, Yuuri was depressed and didn’t know where to go next. It was Minako who grabbed him by the short and curlies and told him to do something and stop moping around the hot springs, because he wasn’t happy and couldn’t hide forever… and, conveniently, her network of friends and collaborators extended to academia, where they were getting increasing traction, and needed a smart, driven person who is already familiar with the supernatural world to really get this rolling. Doing this through a university, rather than independently, gives them access to a lot of resources, including academic databases, the magic of a .edu email address, the capability to actually print and distribute their results, and an aura of legitimacy. 

So, Yuuri is off to grad school, in a department that only kind of exists, pursuing a research project that is heavily side-eyed, even though Japan would be slightly less hostile to the idea of “the myths are real” due to hella different history and mythology than would be found across European cultures with the much more intense pressure of Christianity. 

There, he takes the classes he needs to learn enough about sociological research that he can actually do it usefully, ingratiates himself with related departments, and generally helps Minako’s group convince the Rich Magic People to fund this project, which is basically a test run. There, he meets Phichit, who has graduated from Detroit and, as mentioned in the story, is going for a graduate degree in translation and interpretation, and they obviously hit it off. Then, who honestly can NOT follow Yuuri across the world for such weird shit, because holy charisma, batman.