As a rule of thumb, autumn (October to December) and spring (February to April) tend to be the most congenial times to visit Vietnam.
Vietnam is affected by two monsoons: the southwest monsoon, from March through September, and the cooler northwest monsoon, which blows from October until April. These weather systems have a varying impact on different parts of the country, with the south feeling the brunt of the southwest monsoon and the centre and north receiving their highest rainfall during the winter months. With such complicated weather patterns, it is impossible to pinpoint a single time of year as the best.
Bear in mind that to catch the beach resorts of the south at their sunniest, the dry period between December and March is the optimum period but the north may see some rain may during January and early February.
Average maximum temperatures are much higher in the south, hovering around 27–30°C in HCMC. In Hanoi, temperatures drop to a cool 17–20°C from December to March, peaking at 30°C in the summer months. Higher elevations such as in Sapa, of course, mean lower temperatures.