According to this website
http://aglobalworld.com/international-v ... oltage.php
Vietnam runs on a 110V/220V system. I'm not sure how that is going to break down though. As long as they're plugging into a 110V outlet a simple plug converter should be all thats required. However if they're plugging into a 220V outlet they're going to need a step down converter.
However, according to this website:
http://treehouse.ofb.net/go/en/voltage/Vietnam
They're moving to a 220VAC from a 127VAC system. Given this information it would probably be best to buy a step down transformer. Again voltage converters has what they would need
http://www.voltageconverters.com/voltag ... rters.html They just need to pick the Wattage they require. I swear I don't work for voltage converters, and Amazon sells this same stuff, I just found it was more expensive. If you buy one of those step up/down transformers you just plug it into the 220V outlet, flip a switch on the back the power grid you're using, then plug your stuff in the 110V output. It can also work visa versa if you want to run 220V stuff on our 120V based power grid.
Another option, and potentially easier option, is to buy a new AC adapter that will work with Vietnam's power grid, and output the correct Voltage and Current the DS Lite requires. If you can't get one with the appropriate plug for the DS Lite, you could cut off the DS Lite end from your charger and attach it to the new AC adapter. You could get away with just twisting and taping the wires, but it would be best to solder them.
Its fully well possible that Nintendo makes a DS Lite AC adapter that will both output the correct power to the DS Lite, and work with a 220V 50hz power grid. I would check into a Euro DS Lite Adapter.
Plug Adapter wise, the second website lists the types of plugs Vietnam uses, looks like they use three different types, one of which is our style.