How often does it rain in Vietnam?  

I think something like 65 inches worth rain down in Vietnam per year. I am not sure how this is reflected in days. Sometimes it rains very light and other times it pours. 

You can check the averages here: https://www.vietnamonline.com/weather.html

My notes o the weather in Vietnam :

Because of differences in latitude and the marked variety of relief, the climate varies considerably from the north to the south.

The North:

 -   Perfect in October-November and April-June: temperatures between 21 and 28º. The best periods to visit the North, though, in May, you generally encounter frequent short rainfalls, normally in the evening.

• December to March: rather sunny winter, but cold (12-18º in the delta, 5-12º in the mountains). January and February are not good months because of the cold and frequent thick fog in the mountains (it can go down to 0°C at night in Sapa).
• July to end of September is the “summer” season; 30-40º is the norm ; July is a period of sometimes violent but generally short rainfalls.

Of course, the temperatures are higher and more humid in the delta than in the mountains.

 The Center:

 February to May is the best period, the worst being between September and end of January, when it rains often and heavily (frequent coastal typhoons in September and mainly October, month during which Hoi An is often “under water”. It is noticeably cooler in the Highlands (Dalat, Bon Mê Thuôt) than on the coast.

Dalat, at 1500 m altitude, benefits from a very pleasant climate year around, but you need a warm sweater in the morning and evening in January and February.

 The South:

Temperatures vary less over the course of a year.

-    Best season from December to April, March and April being the hottest months (35º average).

-    Worst period from July to September, very hot and humid, with nearly daily short rains.

If we are talking straight up and down, day by day... I would guess it rains 280 days of the year. I do not know for certain though, I just know it rains a lot even if it is a light drizzle. 

. I would guess it rains 280 days of the year.

Certainly NOT ! Here is the line from Wikipedia Vietnam :The annual average number of rainy days ranges from 60 to 200 in which most days have rainfall averaging less than 5 mm. As I wrote already, it depends on the region and the months.

You can take a guess but it changes from year to year, season to season, region to region. 

It is more wet than other places but not the wettest place you can visit. 

Some places get a lot of rain while other places get average amounts. If you are familiar with the weather in Seattle, Washington then just know it rains there more than some areas of Vietnam. 

I would say overall, the average for the entire country is like 120 days of the year. Some places don't get as much rain as you would think.