VAMA said from January to June its members sold 68,609 vehicles, including cars, trucks and buses, compared with 28,522 in the first six months of 2007.
Japan's Toyota topped the list with 12,510 units sold, followed by domestic companies Vinamotor with 14,594 units and Truong Hai with 11,295.
In June alone, car sales in the fast-industrialising country were up by 60 percent year-on-year at 9,749 units, the association said.
However, auto sales were down 15 percent from May to June after the communist government raised some taxes in a bid to curb imports.
Vietnam, which has struggled with double-digit inflation and a ballooning trade deficit this year, estimated economic growth of 6.5 percent in the first half compared with 7.9 percent in the same period last year.
Prime Minister Nguyen Tan Dung has urged state agencies to limit car imports to curb Vietnam's trade deficit, which widened to an estimated 14.8 billion dollars in the first half, and to reduce city traffic jams.