Peak Season: November to March, July to August
These months see domestic tourists flocking from sleepy beach towns to the big cities and vice versa. Weather is hot and humid, particularly in the south where temperatures can easily top 100°F (38°C). Conditions can fluctuate from gorgeous blue skies to torrential downpours. Christmas and Western New Year are popular with travelers: rates will be higher. Do not travel during Vietnamese New Year (Tet), which falls in late January or early February; it is a madhouse and prices are sky high.
Off Season: November to March
Because of Vietnam's different climate zones, high season in the south is low season in the north. The Mekong Delta region is always hot and humid, but off-season travelers to Hanoi and Halong Bay will find temperatures hovering just about 70°F (21°C) in November, down to the upper-50s °F (upper-10s °C) in January.
Shoulder Season: April to June, September to November
You won't have to fight for accommodations or elbow room at major sites and, because of Vietnam's varying climate, you'll still be able to enjoy peak season weather in some parts of the country. The south is still very hot and it will rain, though not all day. In Hanoi, October and November are the driest months when it is hot and sunny.