some background

Vietnam is a long narrow country. The distance between Hanoi, the capital in the north and Ho Chi Minh City (HCMC) / Saigon in the south is 1760km. For Australians this equates roughly to the distance between Melbourne and Brisbane.
Hanoi features a warm humid sub tropical climate with plentiful precipitation. You will see that bananas grow here but in January 2011 there were some days where the maximun temperature was around 10 degreesC!  The city experiences the typical climate of northern Vietnam, where summers are hot and humid, and winters are, by national standards, relatively cool and dry. Summers, lasting from May to September, are hot and humid, receiving the majority of the annual 1,680 millimetres (66.1 in) of rainfall. The winters are short, relatively dry, and mild, while spring can bring light rains.
Ho Chi Minh City  has a tropical climate, specifically a tropical wet and dry climate, with an average humidity of 75%. The year is divided into two distinct seasons. The rainy season, with an average rainfall of about 1,800 millimetres (71 in) annually (about 150 rainy days per year), usually begins in May and ends in late November. The dry season lasts from December to April. The average temperature is 28 °C (82 °F), the highest temperature sometimes reaches 39 °C (102 °F) around noon in late April, while the lowest may fall below 16 °C (61 °F) in the early mornings of late December.
For more details about the weather in other part see Life and Travel in Vietnam and Vietnam Best time to travel
So when is the best time to visit Vietnam? Commonly, based on the duration of your stay, you should plan to catch the good weather and places by either starting Saigon around February then following the coast to north or hitting Hanoi in October-November then tracing southward. The choice is entirely yours.