The best time to travel to Vietnam depends a great deal on what things to do and attractions most interest you. The country stretches over almost 1,000 miles of latitude, more than the length of California, so the climate varies rather dramatically a great deal. The weather is affected both by the cooler climate coming from China in the north, as well as the warmer tropics in the south. You can experience cold frosty winters in the north, while the south is warm and subtropical almost year round. The seasons in Vietnam are also impacted by widely ranging altitudes—from mountains where there is even occasional snow to the flat, sea-level alluvial plain of the Mekong Delta.
If you think that when to go to Vietnam is not during the rainy season—think again. There are two rainy seasons—in the north from May to October, and in the south from September to January. A Vietnam trip almost any time of year means you will probably encounter some rain if you intend to visit the two most important regions, around Ho Chi Minh City in the south and Hanoi in the north.
Additionally, when to go to Vietnam for the dry season includes the two months during which the Vietnamese New Year is celebrated. This changes from year to year, falling sometime in January or February. For a minimum of the week prior and the week after, virtually every person in the country travels to be with family or on pilgrimages to temples and pagodas. This may be the best time to travel to Vietnam for good weather, but prices are much higher, city traffic is more chaotic than usual, and most of the major sites also have major crowds. A great deal of shopping occurs in the weeks leading up to the celebrations, so even shops are crowded. Then, many stores will close for several days. Many people will want to visit during this colorful festival period, as it is culturally very enriching, but do balance that weather with higher prices and larger crowds.
There is something both positive and negative to be said for a Vietnam trip during any of the seasons. The good news is that the country is extremely beautiful and the people friendly and engaging all year long. Most sites can be enjoyed regardless of the weather. You may want to avoid the south during the hottest months (March through May) although this is definitely when to go to Vietnam if you plan on visiting the beaches. On the other hand, the best time to travel to Vietnam for cruises in Halong Bay is either in the spring or autumn, when it is driest and not as cold.
There actually is some technical mountain climbing in the northern mountains. The highest peak in the country, Mount Fansipan at more than 10,000 feet above sea level, is located here. This region is also excellent for cycling and hiking, and has some fascinating ethnic minorities living in picturesque traditional villages. For a Vietnam trip here, you want to avoid the coldest winter months.