Vietnam

You can make a reference on weatherspark site.
What parts of Vietnam do you plan to visit in March?

Where exactly? It's a very big country.

Burn season occurs during the dry season in the central highland mountains approximately from Mar through Apr. Most tourists don’t visit this part of Vietnam. To learn more about the provinces where this is practiced click on this link: https://en.wikipedia.org/wiki/Central_Highlands_(Vietnam).

Hi, if you planting on going to vietnam, you only need to wearing mask in the main city like Ha Noi, Ho Chi Minh where they have a lot of vehicles around. But if you go to other city, the air is pretty fresh to be honest especially Ha Giang, Lao Cai, Sa Pa where those city locate in the North West mountain or the city near the sea like Nha Trang, Da nang, ... Normally the tour will get you to each of the city for the few day so it wont affect you and you can buy it everywhere in Vietnam with such a cheap price. Glad i can help

We were in Vietnam several years ago -- late February 2019. What I found especially bothersome is the cigarette smoke. Many young men smoked. While far from all, several large trucks had dark exhaust.

While well before the pandemic, we saw cloth masks were commonly sold (and seemed to be worn a lot by local young ladies) in the various street markets. We purchased a few, and I then wore mine anytime I sensed any form of air issues. My husband, however, was not as vigilant, and he came home with a bad sinus issue.

So, my suggestion would be to take masks, and wear them in those certain locations you deem wise. Better to have them and not use them, then vice versa.