It is not overconfident to state that the greatest amount of money for transportation must be the one you pay to get to Vietnam. However, you can surely save up by travelling off season. The relatively low season is between November and March. Most international airlines raise the ticket price significantly around Christmas holiday and during the summer, when travel volume is hard to handle.
Travelling between November and March is also ideal because of the pleasant climate. Do not let the winter discourage you because, in this tropical country, winter is actually much more favorable than summer during which the sunlight and temperature might get pretty annoying. Besides, expenses for accommodation, dinning or inland transportation would also be cheaper during this period.