It was also the first time since September, that the number of new accounts in a month had surpassed 100,000.
The VN-Index gained 3.3% in May to finish at 1,075.17 points.
Investor confidence has improved in the past few months after the State Bank of Vietnam slashed interest rates three times.
Vietcombank Securities Company expects rates to drop further.
The number of new securities accounts was the highest ever in May and June 2022, when over 450,000 accounts were opened before a relentless decline began.