This was the third decline in a row, according to data from Vietnam Securities Depository.
It is 68% lower than the record high figure in May, when over 476,000 new accounts were registered.
In the first eight months, nearly 2.2 million new retail accounts were registered, 43% higher than the 2021 total.
VN-Index inched up 6.1% in August, remaining at the 1,200-point level, far from returning to its historic 1,528-point peak in January.
So far this year, the benchmark has dipped 16.6%.
There are now nearly 6.5 million stock accounts in Vietnam, or roughly 6.5% of the population.