Vietnam is geographically located in the tropical zone, with the top North region on temperate zone. Therefore, its climate is not consistent throughout the country. In fact, the weather varies significantly by latitude and longitude.
The North (Hanoi, Halong Bay) is monsoonal with hot and rainy season from April to September and cool dry season from October to March. October and November are the best months to visit since you avoid the domestic tourist flow and also experience a more comfortable temperature and humidity compared with other months.
The central part of Vietnam (Hue, Quang Binh, etc.) receives dry and hot Southwest wind which makes it strenuous for any brisk walk during the day. Closer to the beach, the level of severity reduced and the dry wind is replaced by cool ocean breeze.
Because of the limited land area, central Vietnam is also most susceptible to natural disasters such as hurricane or flooding. The paradox is, it is also where most beautiful beaches lie.