Following advice is collected from LonelyPlanet Guidebook
Bring as little as possible, Vietnam has pretty much anything you can find back home.
Necessary documents: your passport and visa (with their copies); 2 photos (3cmx4cm), tour dossier (if any) and others (if needed).
Cash in US$5’s, US$10’s, US$20’s and US$100’s (US dollars are commonly acceptable)
All the soaps and smellies are cheap and plentiful, and clothing, shoes and backpacks are all manufactured in Vietnam and available at a fraction of the price in the West. Tampons are available in all major towns and cities, but not in more remote areas.
A Swiss-army knife or equivalent come in handy, but you don’t need 27 separate functions, just one blade and an opener. A torch (flashlight) and compass are also useful.
Other handy things to bring are: name cards, as Vietnamese deal them out like a deck of cards; ear plugs to block the ever-present noise; a universal plug adaptor; a rain cover for the backpack; a sweater for the highlands and air-con bus trips; mosquito repellent to keep the bugs at bay; and a folding umbrella if you plan to visit during the rainy season (July and August are the wettest months).
Finally, the secret of successful packing: plastic bags – not only do they keep things separate and clean, but also dry. That means a lot at the end of a long, wet day.