To ensure there's no fundamental confusion, the terms you're talking about are "nominal" and "parity adjusted", which are what I gave. Nominal is the "absolute" cost of something. Parity is the adjusted value after compensating for how much things cost in a given region. You seem to be proposing that perhaps teachers in Vietnam are paid well relative to other jobs in Vietnam. This is not the case. A fairly recent government study indicated that some 50% [1] of teachers regret deciding to teach as a career, with the article going into detail on the lifestyle of a teacher - which is horrible, in large part due to dreadfully low compensation. PISA measures the performance of 15 year old students, so that is another topic altogether.

This is why Vietnam is such an interesting case study. Like many things, our spending is driven more by intuition than data. And Vietnam shows that our intuition on successful educational outcomes is pretty much useless.

[1] - http://english.vietnamnet.vn/fms/education/25476/50-percent-...