The index closed 13.49 points lower after gaining 7.29 points on Friday. It has dropped 37% since earlier this year.
Trading on the Ho Chi Minh Stock Exchange (HoSE) fell by 14% to VND9.45 trillion ($382.05 million).
The VN-30 basket, comprising the 30 largest capped stocks, saw 20 tickers dropped.
GVR of Vietnam Rubber Group, NVL of real estate developer Novaland Group and PDR of Phat Dat Real Estate Development hit floor price with a 7% decline.
MWG of electronics retail chain Mobile World lost 5.5%, and MBB of lender MB fell 5%.
Other major decliners were VPB of private lender VPBank, BVH of insurance company Bao Viet Holdings and POW of electricity producer Petrovietnam Power Corporation.
Ten blue chips gained, led by SAB of brewer Sabeco with a 3.8% rise.
It was followed by MSN of conglomerate Masan Group, up 3%, and STB of Ho Chi Minh City-based lender Sacombank, up 1.6%.
Foreign investors were net buyers to the tune of VND1.69 trillion, mainly buying STB of Ho Chi Minh City-based lender Sacombank, HPG of steelmaker Hoa Phat Group and SSI of leading brokerage SSI Securities Corporation.
The HNX-Index at the Hanoi Stock Exchange, where mid and small caps list, was down 3.35% while the UPCoM-Index at the Unlisted Public Companies Market was down by 2.64%.