It was sold 0.18% lower from Thursday at VND25,280, having been rising for the last several weeks.
Commercial banks, however, kept the rate unchanged.
Vietcombank sold the dollar for VND24,877. It has gained 8.54% since the beginning of the year.
The rate stayed at VND24,875 at Eximbank and Techcombank.
The State Bank of Vietnam fixed its reference rate at VND23,693, same as Thursday.
The USD Index, which measures its strength against major currencies, rose 0.8% from Thursday to 110.71 points.