There are not too many vegans in Japan, and most of them are foreign nationals. You’d be lucky to meet a vegan Japanese person who isn’t a monk, and I think that in my 4 years of living there, I have only met one vegan Japanese woman. I’ve met a couple of vegetarians as well, but in general the vegan community is non-existent and isolated.
One of the hardest things to come to terms with while living here is dealing with the fact that pretty much no one knows what a vegan is. Everyone is familiar with the term “vegetarian”, but you will definitely have to explain the term “vegan”. People who don’t personally have a vegan friend or acquaintance will probably have never heard the term before. It might be easier to explain that you can only eat “monk’s cuisine”, which is a special category of food reserved for monks who pledge to a diet free of animal products.
Because of this people tend to get confused about what is ok and what is not. I have constantly been asked if fish byproducts or things like gelatine are “OK” to eat even after explaining what “veganism” entails. Japanese people generally have a poor understanding of how certain foods are produced, and they might not consider simple organisms such as shellfish as animal-based.
Most processed and packaged foods are not vegan friendly. You will find egg and milk and fish in the most surprising places. For example oreos in Japan are not vegan, even though they certainly are in America. And most of the time food labels do not provide full disclosure either. Things can be very deceptive because of the more lax labeling rules.
But that does not mean that there is nothing. Lately I see supermarkets stocked with soy-based cheese and ice cream, although spotting these items is rare. There are also a few restaurants here and there that have vegan friendly or completely vegan menus. I recommend the Happy Cow app for searching these places if you are interested as it lists hundreds of restaurants that cater to vegans and vegetarians.
Becuase you will not be able to consume so many processed foods, you may end up spending more on produce and unprocessed things to fuel yourself, but a benefit of this is that you will certainly live a healthier lifestyle as a vegan here, be it voluntary or involuntary.