This is a bizarre case of the largest ISP in a country setting up their network in a way where every customer's IP resolves back to localhost.
There are other hosting providers in the country that didn't make that mistake, but out of the nearly 5 million banners in Shodan for Vietnam 1.5 million of them resolve to localhost.
To give you an idea of how uncommon this is, there are a total of 1,528,188 banners in Shodan that resolve to localhost - Vietnam makes up more than 95% of them:
It's unclear to me why they would chose to do this, so if anybody has any thoughts or insights please let me know.
Edit: This is the search that lead me to the discovery: https://www.shodan.io/search?query=hostname%3Alocalhost