It closed 7 points lower after rising nearly 22 points Tueday.
Trading on the Ho Chi Minh Stock Exchange (HoSE), on which the index is based, surged 29% to VND29.76 trillion ($1.23 billion).
The VN30 basket, comprising the 30 largest capped stocks, saw 19 tickers fell.
VJC of budget airline Vietjet dropped 2.8%, followed by VHM of property giant Vinhomes with a 2.6% decline.
GVR of Vietnam Rubber Group and VIC of private conglomerate Vingroup both dropped 1.8%.
Nine blue chips gained. MWG of electronics retail chain Mobile World went up 3% and VIB of Vietnam International Commercial Bank closed 2.9% higher.
Foreign investors were net sellers to the tune of VND1.2 trillion, mainly selling HPG of steelmaker Hoa Phat Group and STB of Ho Chi Minh City-based lender Sacombank.
The HNX-Index for stocks on the Hanoi Stock Exchange, home to mid and small caps, fell 0.08% while the UPCoM-Index for the Unlisted Public Companies Market dropped 0.10%.