It fell for a fourth month in a row after reaching a historic peak of 476,332 in May.
In the year to date 2.3 million new accounts have been opened compared to 1.5 million in the whole of last year.
The VN-Index fell to a 30-month low last month as ongoing geopolitical tensions and rising global inflation concerned investors.
There are 6.5 million retail stock accounts in Vietnam, equivalent to 6.7% of the population.
But foreign fund managers remain optimistic thanks to the country’s long-term economic prospects.
Investment firms Coeli Asset Management SA and Asia Frontier Capital are looking to increase their positions in the country by buying the dips.
The World Bank expects Vietnam’s economy to expand at 7.2% this year to lead growth among major Asia Pacific countries.