Well, the definition of the word "war" is "A state of open, armed, often prolonged conflict carried on between nation, states, or parties.", so by that definition Vietnam was in fact a war. However, officially in the eyes of the United States government, it was a conflict, because Congress never declared war on Vietnam. In fact, Congress hasn't declared war since 1941, so whenever you hear someone talking of the "war" in Iraq, you should correct them, and if they bring up the definition of "war", note the fact that there is no "nation, state, or party" to be at war with.