The index closed 15 points lower after gaining 10.57 points Wednesday.
Trading on the Ho Chi Minh Stock Exchange (HoSE), on which the index is based, downed 13% to VND13 trillion ($532.67 million).
The VN30 basket, comprising the 30 largest capped stocks, saw 26 tickers dropped.
PLX of fuel distributor Petrolimex lost 5.1%, followed by MWG of electronics retail chain Mobile World with a 4.5 % decline.
POW of electricity producer Petrovietnam Power Corporation dropped 3.1% and CTG of state-owned lender VietinBank closed 2.8% lower.
Foreign investors were net sellers for the session to the tune of VND730 billion, mainly selling VNM of dairy giant Vinamilk and CTG.
The HNX-Index for stocks on the Hanoi Stock Exchange, home to mid and small caps, dropped 0.95% while the UPCoM-Index for the Unlisted Public Companies Market fell 0.77%.