The dong fell by 0.6% to a record 24,511 to the dollar Wednesday, data from Bloomberg shows, after the central bank widened the daily currency trading band from 3% to 5%.
Bloomberg said the Vietnamese currency is set for its longest stretch of losses since June 2008.
The dong has depreciated by around 7.4% this year, according to Vietcombank data.
The State Bank of Vietnam has spent US$23 billion from its reserves defending the currency this year, reducing them to $90 billion, according to an estimate by Malayan Banking Bhd.