The index closed 2.36 points lower after gaining 10.52 points Wednesday.
Trading on the Ho Chi Minh Stock Exchange (HoSE), on which the index is based, dropped 1.85% to VND24.77 trillion ($1.03 billion).
The VN30 basket, comprising the 30 largest capped stocks, saw 16 tickers fell.
VHM of property giant Vinhomes dropped 1.4%, followed by SSB of Southeast Asia Commercial Bank (SeABank) and VIC of private conglomerate Vingroup, both with a 1.3% decline.
HPG of steelmaker Hoa Phat Group closed 1.2% lower.
Eleven blue chips gained, led by GVR of Vietnam Rubber Group with a 1.8% rise.
Foreign investors were net sellers to the tune of VND887 billion, mainly selling HPG and SSI of leading brokerage SSI Securities Corporation.
The HNX-Index for stocks on the Hanoi Stock Exchange, home to mid and small caps, rose 0.31% while the UPCoM-Index for the Unlisted Public Companies Market gained 0.15%.