Trains in Vietnam are very cheap; you can travel from one end of the country to the other and not spend $50. You can get for instance an overnight train from Saigon to Nha Trang for about $15USD; this would be a soft sleeper with 4 people in the cabin. You can save a little more and get a hard sleeper for the same trip for about $12 where you share the cabin between 6 people, and if you are real hard core you can go with a soft seat for around $10, which I would not recommend as these compartments are over crowed with luggage.
Trains also are attractive in Vietnam if you have a lot of luggage as they do not weigh it. You can bring as heavy as you carry and load by yourself and not have to pay over weight limits which is great for scuba divers and surfers. Lets say that you come to Vietnam on a cheap flight from Hawaii, most US international flights allow you unto 70 pounds or 31 kilos; in Asia many flights are 22 kilos max including you hand luggage which they will weigh if you are close to the limit. If you take an overnight sleeper you can also save one night of hotel accommodations which is a great budget stretcher to keep you on the road longer.