We can not say when is the best time to visit Vietnam. Because it always has different areas in Vietnam with a good climate when traveling Vietnam. Therefore, it only depends on which region you are visiting in Vietnam. And the weather in Vietnam is divided into 3 different areas
Northern Vietnam: You will find Vietnam’s capital Hanoi, Sapa, Halong Bay, and Ninh Binh.
Central Vietnam: The endless white sand beaches in Danang, UNESCO protecting Hoi An, Hue imperial city and the world’s largest cave Son Doong are found here.
Southern Vietnam: You can visit the majestic Mekong Delta, fast-growing Ho Chi Minh City, Nam Cat Tien National Park, beaches in Mui Ne and Nha Trang and two exotic islands of Phu Quoc and Con Dao.