I don't know about Japan, but also am curious as to how you'll do there, since my wife and I plan on visiting sometime in the next 3 years.
In the vast majority of Japanese restaurants in Japan there is almost no vegetarian food, unless you want a slab of warmed up tofu, a salad (remember to ask them to hold the bacon / shrimp / chicken) or chips / french fries. If you ask for the gorgeous-sounding dishes you can receive in an
American Japanese restaurant you will be answered with bewildered looks and maybe some giggles.
And it gets worse: even things that
look veggie, like Mexican tortillas, Italian pasta, or even traditional Japanese stuff like okonomiyaki, will probably contain chicken stock. As does just about any prepared meal, soup etc bought in a supermarket.
In Japanese cuisine there are lots of vegetarian recipes, but the Japanese seem to have mostly forgotten about them. Or they just prepare them with chicken stock.
On the bright side, the numerous Indian restaurants here are wonderful. And if you can persuade a Japanese person to prepare some traditional Japanese stuff without chicken stock, as I did in hospital here
you're in for a treat. Especially tasty is veggie tempora, croquets (hold the bacon) and some lovely spicy tofu, daikon and egg dish that my g/friend rustles up for me (can't find it in any restaurants.) And I must add that if you can explain to restaurant staff that you'd like a certain dish without bacon / chicken / shrimp etc they'll try hard to help, although be prepared to be gawped or giggled at. And check out the 'Natural House' chain of supermarkets.
Here's a useful link:
Tokyo Vegetarian Guide: vegietokyo.com