Even though March to May is the best time to visit Vietnam countrywide, there is no bad time to visit Vietnam. It is a huge country with some parts of it always having good weather.
The best time to visit Northern Vietnam (Hanoi, Halong Bay, Sapa)is from October to April. This period experiences cooler, drier weather. October to December offers comfortable temperatures, while January to April is dry and clear, ideal for exploring Halong Bay and trekking in Sapa.
The best time to visit Central Vietnam (Hue, Hoi An, Da Nang) is from February to August. The dry season from February to August is perfect for visiting Hue's historical sites and exploring Hoi An's ancient town without rainfall disrupting plans.
The best time to visit Southern Vietnam (Ho Chi Minh City, Mekong Delta, Phu Quoc) is from November to April. The dry season from November to April offers pleasant weather for exploring Ho Chi Minh City, touring the Mekong Delta, and enjoying the beaches of Phu Quoc Island.