The index closed 4.26 points lower after gaining 12.5 points Tuesday.
Trading on the Ho Chi Minh Stock Exchange, on which the index is based, dropped 17.4% to VND16.85 trillion ($675.5 million).
The VN30 basket, comprising the 30 largest capped stocks, saw 18 tickers gained.
VHM of property developer Vinhomes saw a 2.3% growth, VIC of private conglomerate Vingroup went up 1.7% and FPT of technology giant FPT Corporation rose 0.7%.
Seven blue chips fell. GVR of Vietnam Rubber Group fell 2%, CTG of state-owned lender VietinBank went down 1.8% and MWG of electronics retail chain Mobile World closed 1.5% lower.
Foreign investors were net sellers to the tune of VND625 million, mainly selling VHM of Vinhomes and NVL of property developer Novaland.
The HNX-Index for stocks on the Hanoi Stock Exchange, home to mid and small caps, fell 0.66% while the UPCoM-Index for the Unlisted Public Companies Market went up 0.08%.