We can not say when is the best time to visit Vietnam. Because it always has different areas in Vietnam with the best climate. Therefore, it only depends on which region you want to visit 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, and many charming places
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.