Viet Minh

I'm not sure the South America was the first region in the world to legalise homosexuality actually, even Cuba only legalised it quite recently.

The original Soviet Union under Lenin legalised homosexuality as early as 1917, but Stalin reversed this in 1931.

As for Buddhism and Christianity, generally speaking Christianity can be both more queerphobic than Buddhism and less so. Right-wing Christianity is generally queerphobic, while left-wing Christianity is generally pro-LGBT. Buddhism is somewhere in between left-wing and right-wing Christianity. In theory it supports queer liberation but Buddhists don't actively campaign for queer rights like some left-wing Christians do.

However, Christianity for instance isn't necessarily more queerphobic than Buddhism, because there is Liberation Theology which is generally pro-LGBT. Liberation Theology is quite popular in many South American countries, and semi-socialist countries there like Cuba and Venezuela all officially support queer rights (even though queerphobia still exists in society of course). I think there are probably more queerphobic Christians in the First World than in the Third World.

China isn't really a Buddhist country, but a Confucian one. Though Buddhism is also a major religion.

It's true that Buddhism would seem to be more tolerant of LGBT people on the face of it, since for instance there is a tradition of "3rd sex" people in some Buddhist countries. But socially these people were never treated equally, but more like a semi-social outcast. Therefore it's fundamentally different to the modern secularist and leftist queer movements, in which queer people don't just fight for the right to exist, but actually for the right of complete social equality.