Been working in Education in Vietnam for the past 20 years.
Good to see Vietnam join these rankings and as suspected they have done very well in the maths and sciences. Not sure how they measure reading skills as Vietnamese is totally phonetic, with very simple grammar and a very small vocabulary so any Vietnamese native speaker who goes to school for a few years can obviously read Vietnamese well.
I have never seen a Vietnamese person look up a Vietnamese word in a dictionary. My Vietnamese is not that good, but if there is an English word I don't know then I can often understand it if I am given the Vietnamese equivalent, as Vietnamese words are much simpler to understand.
Obviously they are measuring your skill rather than how difficult it was to achieve that skill.
Vietnamese are highly critical of their education system and they regard the British and American education systems as being vastly superior to their own.
Vietnamese kids study a huge amount and have no time for anything else so most city kids don't know anything that is not in their school curriculum. There curriculum puts a lot of focus on maths and science and gives very little input in the arts, sports, music, etc.. It seems as if the PISA tests were specifically designed for them. Even though the PISA tests suggest Vietnamese students know a lot, I regard Vietnamese as being intelligent, but ignorant.
Believe it is Education New Zealand that says something like, "Education is what you have left when you have forgotten everything you have been taught". By this definition, the Vietnamese education system is seriously lacking.
I feel kids in the UK study too little (or at least waste too much of their spare time on rubbishy activities) and kids in Vietnam study too much. Also feel that what the UK needs to improve its results is parents that are more involved in their children's education, and more emphasis put on getting, training and retaining better teachers.