7 Mission Street Restaurant serves Chinese/vietnamese food. I work a couple of blocks away, and it's a favorite place for lunch. It's really just a little cafe, not at all pretentious, with good food at good prices. The reason I really like it is that the food is above average for this kind of restaurant and price. For under $7 you can get some really tasty, freshly made Chinese food - there's one or two items on the menu that I recognize from Vietnamese restaurants, but mostly it's Chinese.
Try their eggplant chicken, or mango chicken, or clay pot.
It's popular at lunch time - I make a point of going at 11:30 or even 11:45 to beat the rush - and it's noisy, too; once it starts filling up, you can't hear yourself think, much less your lunch companion speak.
It's not in a great part of town (<< understatement), there's no view or other attraction to it, but if you happen to be visiting the Civic Center area for whatever reason - there's 2 -3 hotels around - and are hungry, I recommend it.