Will it be hot and humid in most places?
Northern Vietnam has two distinct seasons - hot and humid summers (vary from 24 to 41 oC) and cool and dry winters (vary from 16 to 23 oC).
Central Vietnam's coastal areas are more temperate than the sticky south (although they do experience heay rainfall), while the Central Highlands are pleasantly cooler. The weather is also hot and reasonably dry from May to September. The monsoon winds then change bringing above average rain during late September.
Southern Vietnam is fairly consistently hot and humid all year round. May to November expect temperatures from 24 - 41 oC, and from December to April, 22 - 33oC.