Time and time again, the central banks of the Emerging Markets proclaim their intent to "preserve stability" or promise to "intervene aggressively" to scare speculators away from their plunging currencies and hold back the tide of capital outflows.

However, when the global macro trend changes (Fed tightening accelerating and balance sheet normalization), there seems nothing the local Central Banks can do to save their precious currency (under who's strength they basked as The Fed puked dollars to the world to save us all for years) as the tsunami of hot capital exoduses faster than it leaked in.

Just ask the Argentines...(who blew a billion dollars on Friday... for nothing)

But that never stops them trying and the latest is Vietnam, where the State Bank has stated its willing to sell dollars at lower price than its current price to calm monetary market and maintain macro-economic stability, according to posting on central bank’s website, citing Pham Thanh Ha, head of its monetary policy department.

Ha said the central bank will closely watch dollar supply and demand in the local market and the development of international markets, ready to flexibly set suitable daily reference rates for VND-USD exchange rates.

And one glance at the chart above shows they have lost control. Nevertheless, Ha claims that the central bank will also take into account impacts from possible US Fed rate hike and China-US trade tension to use other monetary tools and measures to keep local market stable.

Once again the same playbook - remember Argentina threatening "other monetary tools and measures" as if that was somehow the bogeyman that would spook capital flows to stop.

So far that hasn't worked has it?