Vietnam has very complicated weather patterns. Broadly there are two completely different climate zones.
Anywhere south of Nha Trang is hot and dry between November and March and the rainy season is roughly late April to October. Conversely central and Northern Vietnam's rainy, cool season is October to March, and summers (April to September) are hot and steamy.
Central Vietnam is most affected by typhoons; August to November are the most stormy months.