The Dong Xuan Center is a collection of several warehouses which have been converted into a market where you can buy everything from iPhones to knock-off Nike sneakers. In addition to the strange assortment of shopping, though, you'll find several restaurants and grocery stories throughout the warehouses. At the entrance to the warehouse park are two large Vietnamese restaurants, each selling authentic Vietnamese food.
I particularly love the Pho (soup) that's available here, though there are other main dishes are equally good. Prices are a little higher (6-10€ for basic mains) than some of the budget-friendly, all-Asian restaurants in central Berlin, but the quality is exponentially better. You can combine a small shopping spree with a great lunch or dinner. And the market is open on Sundays so it's a great thing to do on the weekend!
In Lichtenberg, Berlin, desolate warehouses renamed after the famed market in Hanoi have really come alive. Eastern Germany's sizable Vietnamese community congregates here on weekends, buying wholesale goods to sell in their shops and feast on sugarcane juice and charbroiled meat, barbecued out in the parking lot. For the past few years, young-and-cool Berliners have caught on, showing up here on trams or bikes in search of really good pho.