I want to go through all the country and not stay in one zone. What is the best period to visit?
March and april are the best months if you want to see the whole country.
Summer months are the worst as it is raining a lot both in north and south and winter can be a little cold up north while fall sees a lot of rain in central Vietnam.
Therefor I would recommend especially march as a good month to go.
From what I've read November is the best time to go, which is when we're going.
Here's my source.