So I was signing up to buy something online and needed the local post / postal / zip code in Vietnam. Within the country, the postal system doesn't rely on post codes. But some international shippers require them, so a halfway lengthy google search (i.e. I didn't hit success on the first search term) gave me Hanoi's code, which is 10999. I dunno if this is correct, because I faintly remember that the system went to a six-digit number sometime in mid-2007, but it could just be some incorrect detritus in my mind.
The info was sourced at TNT, a FedEx/DHL/UPS like delivery service.