The index closed 7.52 points lower after gaining 0.68 points in the previous session.
Trading on the Ho Chi Minh Stock Exchange increased by 31% to VND22.75 trillion ($929 million).
The VN-30 basket, comprising the 30 largest capped stocks, saw 22 tickers fell.
GVR of Vietnam Rubber Group fell 3.8%, and MSN of conglomerate Masan Group dropped 2%.
POW of electricity producer Petrovietnam Power Corporation and VHM of property giant Vinhomes both declined by 1.7%.
The top gainers were MBB of lender MB with a 3.4% rise, followed by ACB of Asia Commercial Bank, up 2.2%.
Foreign investors were net buyers to the tune of VND11 billion, mainly buying STB of Ho Chi Minh City-based lender Sacombank and VCB of state-owned lender Vietcombank.
The HNX-Index for stocks on the Hanoi Stock Exchange, home to mid and small caps, fell 1.03%, while the UPCoM-Index for the Unlisted Public Companies Market went down 0.75%.