There's a halfway decent place 9th and bryant.. sunrise cafe. halfway decent for proximity to where you are going.
Saigon sandwiches is my favorite, tho.
Also near there is pho ha tien 2 on howard @ 5th.
Pho Ha Tien II
937 Howard St, San Francisco, CA
1114 Bryant St, San Francisco, CA 94103
Not really walking distance.....but pretty close by bike, bus, cab or car -
I think they are both open on Sunday during the day, but I'm not positive. Probably worth calling first if you can.
Also, the Latte Express on 5th btwn Mission and Market has good pork banh mi. That's close to Powell BART.
Saigon Sandwich Shop
560 Larkin St, San Francisco, CA 94102
668 Larkin St, San Francisco, CA 94109
48 5th St, San Francisco, CA