But the 0.5% growth in the third quarter-roughly in line with the zone's sustainable rate-seems to have been built largely on domestic demand rather than net exports.

Mr Bernanke said output growth could be "modestly below" its trend rate in the very short term, but made clear that he expected the economy to be growing at its sustainable rate within a year.

So a modest weakening in external demand might be just enough to stop the economy overheating and curb inflation, allowing Vietnam's remarkable run of growth to continue at a more sustainable rate.