Upvote:2
Northern Vietnam and southern Vietnam have different climate patterns. Hanoi area, since you mentioned Ha Long Bay, has a rather drizzly spring, even though it is still considered "dry" season. Ho Chi Minh on the other hand is more sunny at that time of year.
But this year has been different, as in more rain, throughout SE Asia. The southern portions of Thailand and coastal areas in other parts of SE Asia have had much more rain than normal, many areas going for days without sunshine. Even here in northern Thailand where we see basically no rain for months, has had a number of unexpected rainy days.
Upvote:2
It very much depends on which area of Vietnam you're talking about. Mountains, coastal flows, even rivers and caves can have their own climate.
From at least one travel guide online, you can see how it differs from region to region.
Northern - November to April seems to have more chance of cloud, and rain
Central - similar with monsoon type weather from Nov to March, although it can depend on the town (see link)
Southern - May to November is raining season (note how it's almost the opposite of the north).
At the bottom of the linked page are charts for climate, rainfall for each month for the regions, followed by links to weather for individual cities.