Vietcombank sold the greenback at VND26,150, down 0.12% over the weekend. On the black market, the currency fell 0.19% to around VND25,950.
The State Bank of Vietnam lowered its reference rate by 0.02% to VND25,635.
Globally, the dollar inched higher to hold near a two-month high on Monday, as the US-Iran standoff continued to push up oil prices while investors looked ahead to a data-packed week for more clues on inflation and central banks' moves, Reuters reported.
The dollar index, which measures the US currency against a basket of peers, was a touch higher at 101.15 and on track for a 1.7% gain in September, its best month since June.
The euro and sterling were both 0.1% weaker against the dollar, last at $1.1379 and $1.3232, respectively, hovering near multi-month lows against the greenback. The yen was last down 0.3% at 157.7 per dollar.
"The greenback could overshoot in the near term if energy market tensions persist and inflation risks continue to build," said Sim Moh Siong, FX strategist at OCBC.