The Ho Chi Minh Stock Exchange's VN Index fell by midday as investors sold shares on worries about bad debts, inflation and corporate performance, analysts said.
The index was down 1.07 percent to 388.36 at 0430 GMT.
"Shares fell as individual investors sold to protect their assets, while institutional investors have not returned while they were not certain about market trends and after Moody's rating downgrade," Vu Thanh, an analyst at ACB Securities said.
Investors were worried about bad debts in the banking system, particularly those held by state-owned companies, Thanh said.
The index could re-test the 380-point threshold in the near term, he added.
"Consumer prices rising sharply in September have made investors worried about a vicious circle ahead," Vu Thi Thu Trang, an analyst at APEC Securities said.
Domestic businesses have been struggling with difficulties and their third-quarter earnings may not be as good as in the April-June period, she said.
1150 (0450 GMT)
(Reporting by Vu Duy; Editing by Ho Binh Minh)
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Keywords: MARKETS VIETNAM STOCKSNEWS/