Vietnam is a long, narrow country and weather varies significantly between north, center, and south. There is no single "best time" that works for all regions simultaneously.
North Vietnam
The north (Hanoi, Halong Bay, Sapa) has four distinct seasons. Spring (March–April) and autumn (September–November) are the most comfortable, with mild temperatures and reasonable visibility. Summer (May–August) is hot and humid, with occasional typhoons. Winter (December–February) can be cold and grey, especially in the mountains — Sapa sometimes sees frost. Halong Bay is bleakly atmospheric in winter mist but can be choppy.
Central Vietnam
The center (Hue, Hoi An, Da Nang) has its wet and dry seasons reversed from the south. The dry season runs from February to August, with clear weather and beach conditions at their best. The wet season (September–January) brings heavy rain and occasional typhoons, especially October–November when flooding around Hoi An is common. The shoulder months of February–March are a sweet spot.
South Vietnam
The south (Ho Chi Minh City, Mekong Delta, Phu Quoc) has a simpler split: dry season November–April, wet season May–October. The wet season doesn't mean constant rain — downpours are typically short and heavy, and mornings often stay clear. The driest months are December–February and that's peak season for beach destinations like Phu Quoc.
Practical Advice
Tet (Lunar New Year, usually January or February) is Vietnam's biggest holiday. Prices rise sharply, many businesses close, and transport books out weeks in advance. Millions of people travel home to their villages. It's worth experiencing if you're prepared for the disruption, but don't plan on smooth logistics.
If you're doing the classic north-to-south (or reverse) route, February to April and September to November tend to work reasonably well across most of the country. Avoid peak holiday periods unless you book well ahead.