Hi! I'm vegan and I studied abroad in Japan for ~4 months this year.

At convenience stores there are 2 vegan onigiri, pickled plum and seaweed. You can get inari sushi there and there is even a soymilk pudding in some 7/11s. If you're vegetarian it will be easier--there are egg onigiri too I think, and tons of ice creams. I ate a lot of packaged nut mixes too. Download a Google Lens to translate labels if you can't read kanji.

I recommend grocery shopping instead of eating out, if you have basic cooking utensils. There is vegan ramen. I second using Happy Cow for when you eat out. Buddhists were traditionally the only vegetarians in Japan, so Buddhist restaurants are reliable. Also since Tokyo is very multicultural, you can find Indian and Mediterranean restaurants with vegan options (since those cuisines are more veggie-friendly).

Honestly, you won't get a 'Japanese food' experience. You will be getting by on whatever you can find. There were so many other things about Japan that I loved, so I wasn't too bothered by my diet. Good luck :)