Londonist is quite fond of this little budget place called Viet, one of the few Vietnamese restaurants not in East London, and instead is located very conveniently in Soho. Skip their Thai offerings and go straight for their Vietnamese dishes such as their bun, cold rice noodles with lots of fresh raw herbs and vegetables and some form of grilled protein or deep fried treat on top. Also tasty are their noodle soups - pho and bun bo hue - the former being thin flat rice noodles in a beefy (or chickeny if it's pho ga) stock while the latter is thicker round rice noodles in a spicier broth. Both are also served with plenty of the fresh herbs and raw beansprouts. Or if rice is your thing, try one of their various rice plates, such as the shredded pork and crab meat cake below. Service can by iffy but when the food's this cheap, it's hard to quibble.
34 Greek Street, London, W1D 5DJ