Last year Vietnam gave nationals of some countries a 15-days visa exemption. I have been looking for information about the conditions to apply but I can't find anything referring to which are the valid entrance points.
Last April I entered Vietnam without problems through the Ho Chi Minh international airport without a previous visa and for free, but I had to explain to the agent that Spanish citizens have an exemption if traveling less than 15 days. He asked someone and agreed to let me in.
This and the fact that sometimes this kinds of regulations apply only to airports entries, makes me wonder if it's possible to enter Vietnam without paying for the visa by a land border (in my case from Cambodia).