Vietnamese in the Valley
There's a small Vietnamese community in the Van Nuys and Reseda area. Both locations of Pho So 1 are pretty good; I prefer the one on Reseda as opposed to the one on Sepulveda but I can't ever put a quantitative reason why: it just "feels" better. But both are good, and are the best in the SFV. Nearby the Reseda location are Vietnamese grocery stores and banh mi shops.
There are also some pho shops in North Hollywood as well, but your best bet will be to stick to the Reseda/Van Nuys area.
Don't forget to explore beyond just the beef noodles. Some of my favorite Viet dishes are the appetizers like cha gio and nem nuong. Go with a bunch of friends so you can all share the appetizer dishes.
Also, if you get a little "noodled" out, order banh cuon dishes (where you roll your own rice-paper) or com tam dishes (made with "broken rice").
And of course the drinks. I'm a fan of ca phe den da, but that's because the condensed milk in ca phe sua da gets waaay too cloyingly sweet. Both will keep me alert for days. I know the Reseda location of Pho So 1 also has durian milkshakes, if you want to be a little adventurous.
There are two pho shop chains in the Valley -- Pho So 1 and Pho 999. Pho So 1 has locations at Sherman/Reseda and Victory/Sepulveda; Pho 999 has an additional one at Sherman/Coldwater. There are a couple of banh mi shops at Sherman/Reseda.
The pho shops sell bun and a couple of com tam plates, as well as hu tieu, mi and stirfried dishes. Pho So 1 in particular has a very tasty bun tau hu ky (vermicelli salad with shrimp-stuffed tofu).
I'm not big on Pho, but there's a great Banh Mi shop on Sherman Way, a block or so west of Reseda Blvd, called Sandwich Express. Their baguettes are baked in-house, and are fantastic. They have a bunch of exotic-looking side dishes and such, but I always stick to the sandwiches.
Around the corner, on Reseda Blvd, is another Banh Mi shop, La Baguette, which has great BBQ pork, but the baguettes are not as good -- store-bought, I think.