There is no neither ideal nor bad time to visit Vietnam as there are distinct regional weather variations between the Northern, Central and Southern regions.
January & February
The weather in north and along the coast of Vietnam in Jan and Feb is very comfortable, and among best months to travel of the year.
March, April and May
March/ April are pretty good too and less busy. It will start to rain more in May, but not all day. Phu Quoc Island is pretty pleasant up till September, when it rains all day. March is one of the best month to travel as it is one of the driest times of the year, although there can be occasional rain showers around Hue and Hoi An.
June-Aug are summer months and extremely hot in northern & central cities. It’s also the wet season in HCMC and the central highlands (and Phu Quoc in the south) and it’s the hottest time of year in Hanoi.
November & December
Nov and Dec are good months to visit Vietnam. not so cold in northern Vietnam at that time, Hanoi about 15-25oC, while Sapa about 10-20oC. The coldest months are Jan and Feb. Central and southern Vietnam are great at that time (except typhoon). The November visit was by far the most comfortable in terms of temperature and humidity (or lack of).
South Vietnam (HCMC and Mekong Delta, Can Tho, Chau Doc), the weather is fine, a little warmish and sticky, but not at all oppressive. No rain, only a few evening dry thunderstorms, very beautiful over the Mekong.