When last in San Francisco I tried a few restaurants along Irving Street's Ninth to Twenty-Fifth Avenue segment. While there is only one vegetarian restaurant along that mile, several restaurants offer menus that will make vegetarians and vegans happy.
The vegetarian restaurant, which I've not tried, is Shangri-La, at 2026 Irving (between 21st and 22nd, north side of the street), 731-2548, a Chinese vegetarian restaurant "under kosher supervision" that has sworn off use of MSG and is open every day from 11:30 to 9:30. Lots of vegetal meat and fowl on the menu. The restaurant claims to have been there since 1978.
Promising looking is New Eritrea, 907 Irving (between 10th and 11th, south side of the street), 681-1288. Not a vegetarian restaurant, but if it's true to the type, the vegetarian dishes are far tastier than the alternative.
A good Taiwanese restaurant is Taiwan Point Restaurant, 1920 Irving (between 20th and 21st, north side of the street--look for big orange and black sign), 665-9879. The characters on the Chinese sign read Yi1 Jia3 Tian2. Open every day 11 to 11. I tried the rice porridge with yams, the green onion pancake (exceedingly thin, like no cong you bing I've ever had, but delicious), the Taiwan style fried A choy (A cai4) and the eggplant with garlic sauce (sweet), all of which were good. Also on the menu are soy milk, fried Blue Lake beans, fried mushrooms, "special green onion pancake" (zhua bing), fried yin2 si1 juan3, red bean dumpling soup, soybean delight (san1 se4 mao2 dou4), simmered cabbage, bean curd with brown sauce, on choy (kong1 xin1 cai4) with garlic sauce, marinated cucumbers and plenty of items with meat and seafood.
Further west on Irving is the small Palestinian Sunset Deli, on the south side of the street, where an amiable fellow makes good falafel sandwiches.