Recently I went to two places Yokohama: one is in Chinatown and is a Taiwanese restaurant with a vegetarian and vegan-friendly menu, and the other is close by, in Kannai. Both are ideally located, close by to train and subway stations in central Yokohama.
The first is a Taiwanese restaurant called Banwaro. Banwaro is a small restaurant run by a friendly couple. They have a non-veggie menu and a number of items suitable for vegetarians and vegans–just be sure to ask! English appears to be spoken here, as well.
I ordered a stir-fry, soy meat shrimp dish, and gyoza that were all delicious!