It closed 1.8 points lower after rising by the same amount Wednesday.
Trading on the Ho Chi Minh Stock Exchange, on which the index is based, dropped 14% to VND20 trillion ($786 million).
The VN30 basket, comprising the 30 largest capped stocks, saw 16 tickers fell.
SAB of brewer Sabeco dropped 2.2%, followed by VNM of dairy giant Vinamilk with a 1.9% decline.
GAS of state-owned Petrovietnam Gas and HDB of HDBank both fell 1.7%.
Thirteen blue chips gained, led by BCM of Becamex Investment and Industrial Development with a 6.4% surge.
Foreign investors were net sellers to the tune of VND1.7 trillion, mainly selling VHM of property giant Vinhomes and DGC of Duc Giang Chemicals Group.
The HNX-Index for stocks on the Hanoi Stock Exchange, home to mid and small caps, rose 0.03% while the UPCoM-Index for the Unlisted Public Companies Market went up 0.37%.