Given Vietnam current development? The National Bank just have to stop its plan to keep Dong at a low value. However, this is unlikely to come to past in the near future due to the fact that a low value Dong is beneficial to the key demographic of their policy: those who earn their living in foreign currency, then convert it into Dong.

Hypothetically speaking, that mean once the conversion is made, you become a high end earner while the government can stored foreign currency. Win win.

Vietnam is a developing nation, and a fast one at that. If left to its natural course, the value of Dong should naturally increase in value slowly. However, the National Bank policy is to create small scale, controlled inflation through periodically printing more money and release it into circulation.

So with the current environment of Vietnam economy, the surest way for the Dong to increase in value is for the National Bank to stop its current policy of a weak Dong.