It depends on the regions you visit. The north of Vietnam is extremely hot and humid from June to August. The winter is from November to March and it quite cold especially in mountainous areas including Sapa and Mai Chau.
The Central Vietnam is warm throughout year. The rainy season usually start from August to December. The storms and typhoons usually reach these nearby regions in October and November.
The south of Vietnam has two distinct seasons: dry season and wet season. Wet season runs from June to until October while the dry season last from November to May.
Depend on the time and region you plan to visit Vietnam that you should pick necessary items to bring to Vietnam.