The index closed 1.4 points lower after gaining 15.9 points Wednesday.
Trading on the Ho Chi Minh Stock Exchange (HoSE), on which the index is based, dropped 12.6% to VND15.9 trillion ($651.77 million).
The VN30 basket, comprising the 30 largest capped stocks, saw 18 tickers fell.
SSB of Southeast Asia Commercial Bank (SeABank) dropped 4.8% and STB of Ho Chi Minh City-based lender Sacombank fell 3.3%.
VRE of retail real estate arm Vincom Retail closed 2.5% lower and SAB of brewer Sabeco declined by 2.3%.
Ten blue chips gained, led by PLX of fuel distributor Petrolimex with a 3.3% increase.
Foreign investors were net sellers to the tune of VND509 billion, mainly selling STB of Ho Chi Minh City-based lender Sacombank and GMD of logistics company Gemadept Corp.
The HNX-Index for stocks on the Hanoi Stock Exchange, home to mid and small caps, fell 0.57% while the UPCoM-Index for the Unlisted Public Companies Market dropped 0.33%.