I think this came up once before. The species is Ochna integerrima and its native range takes in most of mainland SE Asia. I have seen it growing wild in central and eastern Thailand, e.g. in foothills of Khao Yai near Nakhon Nayok.

I think that in Vietnam O. integerrima is regarded as something like the national flowers, which may be why you heard that it comes from there.

Here are some pics I took in January last year of O. integerrima growing wild near Nakhon Nayok. As you can see, this was the height of the dry season.