The only vegetarian friendly place I found was Mumokuteki in Kyoto, Japan. My biggest recommendation is Lawsons (similar to a 7-11), most of the food has english subtitles making it much more reliable. Its also a lot nicer than a 7-11 in the US, the food is actually very good.

Beyond that just be prepared to explain over and over again what you can or can't eat. Do not trust anyone who seems to understand the first time, it's not a concept that translates well and servers trying to please will say yes and then serve you beef. Street food was also pretty reliable you'll see a fair amount of Tofu and Burdock root by itself, both are pretty good.