The index closed 8.19 points lower after gaining 6.38 points on Wednesday.
Trading on the Ho Chi Minh Stock Exchange (HoSE) fell by 7.87% to VND9.22 trillion ($391.26 million).
The VN-30 basket, comprising the 30 largest capped stocks, saw 23 tickers dropped.
VJC of budget airline Vietjet dropped 5.6%, followed by VHM of property giant Vinhomes with a 2.8% fall.
PDR of Phat Dat Real Estate Development lost 2.4% and STB of Ho Chi Minh City-based lender Sacombank fell 2.2%.
Seven blue chips gained, led by PLX of fuel distributor Petrolimex with a 1.9% rise.
GVR of Vietnam Rubber Group rose 1.4% and GAS of state-owned Petrovietnam Gas went up 0.7%.
Foreign investors were net buyers to the tune of VND18.87 billion, mainly buying STB and HPG of steelmaker Hoa Phat Group.
The HNX-Index at the Hanoi Stock Exchange, where mid and small caps list, was up 0.14% while the UPCoM-Index at the Unlisted Public Companies Market was up by 1.07%.