The index closed 9.95 points lower after gaining 2.41 points on Wednesday.
Trading on the Ho Chi Minh Stock Exchange (HoSE) increased by 22.72% to VND15.79 trillion ($673.29 million).
The VN-30 basket, comprising the 30 largest capped stocks, saw 22 tickers dropped.
GVR of Vietnam Rubber Group led with a 3.7% fall, followed by SSI of leading brokerage SSI Securities Corporation, down 2.9%.
STB of Ho Chi Minh City-based lender Sacombank lost 2.3% and POW of electricity producer Petrovietnam Power Corporation dropped 2.2%.
Seven blue chips bucked the trend.
NVL of property developer Novaland Group rose 3.1%, followed by TPB of private TPBank, up 1.8%.
Foreign investors were net sellers to the tune of VND219.9 billion, mainly withdrawing from KDH of real estate firm Khang Dien House and STB.
The HNX-Index at the Hanoi Stock Exchange, where mid and small caps list, was down 0.54% while the UPCoM-Index at the Unlisted Public Companies Market was up by 0.77%.