The alternative, used by many foreigners, is to charter a minibus. They cost more but they are much more comfortable. You'll find people taking bookings in popular tourist destinations. Budget hotels and cafes are the best places to start looking. 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. Travelling through Vietnam 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. The Vietnamese government has announced plans to phase out cyclos in a cosmetic bid to improve Vietnam's image. If you're in a hurry and have nerves of steel, try flagging down any passing motorbike.