The index closed 6.04 points lower after gaining 1.21 points on Tuesday.
Trading on the Ho Chi Minh Stock Exchange (HoSE) increased by 1.52% to VND9.72 trillion ($413.88 million).
The VN-30 basket, comprising the 30 largest capped stocks, saw 15 tickers gain.
GVR of Vietnam Rubber Group lost 2.9%, followed by VPB of private lender VPBank with a 2.7% decline.
POW of electricity producer Petrovietnam Power Corporation dropped 2.2% and SSI of leading brokerage SSI Securities Corporation closed 2.1% lower.
Five blue chips gained, led by SAB of brewer Sabeco with a 2.8% gain.
MWG of electronics retail chain Mobile World rose 1.4% and VJC of budget airline Vietjet went up 1.1%.
Foreign investors tuned net sellers to the tune of VND460.02 billion, mainly withdrawing from Vietnam Maritime Commercial Joint Stock Bank (MSB) and BMP of Binh Minh Plastics.
The HNX-Index at the Hanoi Stock Exchange, where mid and small caps list, was down 1.15% while the UPCoM-Index at the Unlisted Public Companies Market was down by 0.79%.