Most fares, if you can brave the ride, are ridiculously cheap. There are express buses, but few are really fast. The alternative, used by many foreigners, is to charter a minibus. They cost more but are much more comfortable. Train travel can be even slower than bus travel, but it is more relaxed and you're likely to have decent legroom.
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 and nerves of steel. Travelling through Vietnam by bicycle is worth considering, though the traffic is still a hazard on highways without wide shoulders.