Vietnam Airlines has a near-monopoly on domestic flights, which are relatively expensive. The departure tax on domestic flights is about $US1.50, payable in Vietnamese dong only.
Ultra-cheap buses and minibuses criss-cross the country in an impressive network of routes but you should think long and hard before taking one. Apart from being ramshackle, extremely slow and hugely overcrowded, the notion of safety on Vietnam's roads is a loose and hazy concept that doesn't bear too much investigating. There are 'express' buses, but even these rarely average more than 50kmh (31mph). The alternative, used by many foreigners, is to charter a minibus. They cost more but are much more comfortable; ask at budget hotels and cafes for details.
While sometimes train travel can be slower than bus travel, it is safer, more relaxed and you're likely to have decent legroom. There are several types of train, including the famous Reuinification Express; but think twice before you take a crowded, snail-paced local train. Petty theft can be a problem on trains, especially in budget class. Children throwing things at carriages, everything from rocks to cow dung, is another problem, and you're advised to keep the metal shield on the window in place.
Hire cars and drivers are available at reasonable prices. You'll still be stopped by the police to pay all sorts of 'fines', but at least you'll have a local with you to do the negotiating. You can hire a motorcycle to drive yourself if you have an International Driver's Permit endorsed for motorcycles, but you'll need nerves of steel.
Travelling through Vietnam, and around the towns and cities, by bicycle is worth considering, though the traffic is still a hazard on highways without wide shoulders. Trains and buses will carry your bike when you want a break.
Other than a few ancient and infrequent buses, local transport is by taxi (some metered, some not) or cyclo (pedal-powered vehicles that are cheap and plentiful). If you're in a hurry and have nerves of steel, try flagging down any passing motorbike. Many people will be happy to give you a lift for a fee a little higher than the equivalent cyclo fare.
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