Tbh I mostly just survived because I was vegetarian back then, I went vegan some time after I got back. You can definitely do it, just use happy cow and plan well. Also you can live on kombu and pickled plum onigiri from 7/11 lol.

Also if you get out to Kobe, you should visit a place called "Vegan Cafe Kiu." One of the best meals I had on the entire trip and the coziest cafe I've ever been in, pretty sure it's just a house.