The index closed nearly 15 points lower after losing 7 points Wednesday.
Trading on the Ho Chi Minh Stock Exchange (HoSE), on which the index is based, dropped 9% to VND27 trillion ($1.11 billion).
The VN30 basket, comprising the 30 largest capped stocks, saw 22 tickers fell.
VIC of private conglomerate Vingroup dropped 6.3%, followed by VHM of property giant Vinhomes with a 6% decline.
SSB of Southeast Asia Commercial Bank (SeABank) fell 3.1% and FPT of IT giant FPT Corporation closed 2.1% lower.
Six blue chips gained, led by VIB of Vietnam International Commercial Bank with a 3.3% rise.
Foreign investors were net sellers to the tune of VND209 billion, mainly selling VHM and HPG of steelmaker Hoa Phat Group.
The HNX-Index for stocks on the Hanoi Stock Exchange, home to mid and small caps, fell 1.66% while the UPCoM-Index for the Unlisted Public Companies Market closed 0.56% lower.