March is a great time to be in Vietnam in general.
Vietnam’s southern beach resorts including Nha Trang, Mui Ne, Phu Quoc and Con Dao at this time of year normally see long sunny, cloudless days with little rain.
Saigon and the Mekong Delta see similarly settled and ideal conditions.
Hoi An is also usually great at this time of year with temperatures in the mid to high 20’s. Rainfall is much lower than in the preceding 3-4 months, although the occasional late afternoon downpour can happen at any time.
In Hanoi, temperatures are lower in March than in Central and Southern Vietnam but generally warm enough not to need warm weather gear, especially in the latter half of the month.
Cruising on Halong Bay is beautiful at any time of year and March is no exception. The weather is normally warm enough at this time of year for comfortable kayaking although early mornings on deck can still be a bit chilly.
Those heading into the mountains north of Hanoi should be prepared for cold conditions in the Winter and hot days but chilly nights and early mornings in the summer.