The index closed 8.22 points lower after rising 1.23 points on Wednesday.
Trading on the Ho Chi Minh Stock Exchange (HoSE) increased by 31% to VND23.69 trillion ($1 billion).
The VN-30 basket, comprising the 30 largest capped stocks, saw 23 tickers fall.
SSI of leading brokerage SSI Securities Corporation and VIB of Vietnam International Commercial Bank both lost 3.2%.
GVR of Vietnam Rubber Group dropped 3% and MWG of electronics retail chain Mobile World closed 2.6% lower.
Six blue chips gained, led by VCB of state-owned lender Vietcombank with a 3.1% increase.
Foreign investors were net sellers to the tune of VND312.65 billion, mainly selling VNM of dairy giant Vinamilk and GEX of electrical equipment maker Gelex Group.
The HNX-Index at the Hanoi Stock Exchange, where mid and small caps list, was down 1.54% while the UPCoM-Index at the Unlisted Public Companies Market was down by 0.64%.