Vietnam has a varied climate, partly because it is a long country, so you should consider the region you want to visit as well as the time of year. The north, for instance, is the coldest part of the country, and the coldest months are from November to March. As Vietnam has a warm, humid climate overall, many visitors like to visit the north during these months - the typical temperature is around 17 degrees celsius, which for northern Europeans, for instance, is quite pleasantly warm. It gets hot again in May, and the rainy season starts again in September.
Central Vietnam is drier and hotter - it often has colder winters than the north, but then it can also get wetter, which isn't always attractive to visitors. Dalat and Pleiku have a relatively fresh and cool climate even in summer, but elsewhere the region may be too hot in summer.
South Vietnam can get very hot, and even local people complain that it gets humid. Winter and early spring are the most popular times to come to this part of the country.