Be careful, the climate of Vietnam is sometimes capricious and varies according to the regions, North, Center and South.
For an adventure in the famous caves of the country, avoid the period from October to February since the rains make the areas more slippery and prevent the complete discovery of the caves.
For Dalat, it is better to go there from November to May, before the rainy season.
For Northern Vietnam, the period from April to October is ideal, and when the weather is clear, you can enjoy the intense feeling of emptiness on the glass bridge of Sapa.
To go scuba diving in Hoi An, it is necessary to privilege the period from March to September to have more visibilities and to fully enjoy the incredible seabed.
Nevertheless note that the weather is still an uncertain element, so before you care about the period, think about what you like, So even in rainy weather you can experience extraordinary adventures in the land of the Dragon and thrill with pleasures during a unique experience in incredible settings.