The index closed 1.19 points lower after gaining 9.78 points on Friday.
Trading on the Ho Chi Minh Stock Exchange (HoSE) increased by 13.76% to VND13.62 trillion ($580.81 million).
The VN-30 basket, comprising the 30 largest capped stocks, saw 15 tickers drop.
GVR of Vietnam Rubber Group fell 3.9%, followed by NVL of property developer Novaland Group and SSI of leading brokerage SSI Securities Corporation, both with a 1.5% decline.
VCB of state-owned lender Vietcombank closed 1.2% lower and BVH of insurance company Bao Viet Holdings lost 1.1%.
Thirteen blue chips gained, led by VIC of private conglomerate Vingroup with a 5.2% rise.
TCB of the largest private lender Techcombank closed 2% higher, and VIB of Vietnam International Commercial Bank went up 1.7%.
Foreign investors were net sellers to the tune of VND376.52 billion, mainly selling VIC and VHM of property giant Vinhomes.
The HNX-Index at the Hanoi Stock Exchange, where mid and small caps list, was down 0.36% while the UPCoM-Index at the Unlisted Public Companies Market was up by 0.54%.