The Uptown neighborhood---Argyle/Broadway/Sheridan---Argyle red line stop(Li'l Chinatown nee' Li'l Vietnam)is your destination: banh mi are found many places among them Tank Noodle, Ba Le, and the BBQ/banh mi place along Sheridan(whose name escapes me at the moment...Vinh Phat? is that it?).
In addition to the aforementioned establishments, both Pho 888 and Pho Xe Lua offer an array of delicious banh mi. Pho 888 makes their own cha lua, or pork roll, so the banh mi with cha lua is a good bet. And, Pho Xe Lua makes a delicious mixed pork banh mi with plenty of creamy butter which is a good bet, too. Oh, and like it's close relation Pho Xe Tang, or "Tank," as it's more widely known, Pho Xe Lua makes a great banh mi with warm sardines.
1137 W. Argyle
Pho Xe Lua
1021 W Argyle