The index turned towards red in the afternoon and closed nearly eight points lower.
Trading value on the Ho Chi Minh Stock Exchange (HoSE), on which the index is based, rose 11.4 percent to VND35.41 trillion ($1.56 billion).
The VN30 basket, comprising the 30 largest capped stocks, saw 22 tickers in the red, led by HDB of HDBank and TPB of private TPBank, both falling 3.5 percent.
SAB of brewer Sabeco dropped 2.9 percent to a one-month low.
SSI of leading brokerage SSI Securities Corporation ended a chain of four gaining sessions with a 2.7 percent loss.
Other losers included PNJ of Phu Nhuan Jewelry and CTG of state-owned lender VietinBank.
Seven blue chip tickers bucked the trend with VIC of biggest private conglomerate Vingroup leading with a 4.8 percent gain.
VPB of private lender VPBank rose 3.2 percent to the highest since July 7. This was its third gaining session in a row.
Foreign investors were net sellers to the tune of VND2.1 trillion, the highest since mid-January. They focused on selling VPB and HPG of steelmaker Hoa Phat Group.
The HNX-Index for stocks on the Hanoi Stock Exchange, home to mid and small caps, fell 0.23 percent while the UPCoM-Index for the Unlisted Public Companies Market lost 0.24 percent.