huh after experiences otherwhere in Asia, Vietnam is very void of bugs, compared to Thailand for example. Someone told me it's mainly because of napalm in the war. Who knows
My experience is that it is because of the high number of swallow bird houses.
If you drive around (I've driven most of the southern end of VN and Cambodia), the multi story, windowless buildings are everywhere and in towns with a lot of them, far fewer bugs. The Vice article quotes 4200 houses... my guess is that it is 10x higher than that.
I'm currently in Hoi An and there is a ton of bugs here and not as many bird houses.
http://vietnamnews.vn/sunday/features/216083/swallow-nest-fa...
https://munchies.vice.com/en_us/article/ev8da4/farming-edibl...