Agreed that it certainly can feel "fluked" or random which areas are overwhelmed and which recover, but I have to disagree with the idea that Vietnam didn't take it particularly seriously.

I was there in February, for around a month.

As a foreigner, I was regularly receiving SMS updates about covid infections. There was a website which detailed exactly when and where people were testing positive. Hotels and guest houses, if allowed to remain open, were being closed for deep-cleaning.

Buses and trains were stopped and my temperature checked, towns and islands were quarantined where someone tested positive, contact tracing appeared to be in full force and foreigners were requested to register their movements and check in daily(iirc) to a web portal.

Again, this was in mid-February.

Luck and climate seem to be factors, but Vietnam acted when they had a low caseload and handled outbreaks sufficiently well to keep R below 1. It wouldn't be all that surprising to me if there really were no covid deaths there.