Cheapest month to travel?  

The cheapest months overall are April, September, and October. These months are considered off-season so prices come down a bit but you have to be aware of the weather and pick your location accordingly as these months tend to be fairly warm and humid with lots of rain.

I think Match and April tend to be the cheapest but it really depends on flights and what is happening during the year. Prices have gone up from COVID but they are starting to come back down again.

It seems to vary from year to year. I would keep an eye out on prices now to give you an idea and check what they were throughout this year. You can signup for deal trackers too that predict the best time to buy tickets at the lowest costs.

September tends to be the cheapest but this really depends on where you live. I would say September and April both are good months. 

The best months are in the offseason which is March and April, and again in the fall in September and October. These are the times when you will see the cheapest flight prices and hotel stays.

Certain NOT April, which is one the few best months to visit with June and October. I would say November to end of February (except the 2 weeks of Têt). Now, it also depends on the region climate. In the South, Jan and Feb are expensive because best weather, but less expensive in the north because of the cold. As a matter of fact, the best Halong boat operators consider the high season in the North from 1 May to 30 September, Why not October also remains a mystery to me. 

Ideal because perfect weather everywhere, neither cold or very hot

The cheaper months tend to not be ideal for weather. I have seen lowest prices with the best weather in terms of Vietnam in October/November and February. 

I repeat : APRIL is perfect weather EVERYWHERE