The Vietnamese, who stood second in the qualifying round, triumphed on Saturday in the women's vault event with her result of 14,525 points. Giulia Steingruber of Switzerland came second (14,413) and Teja Belak from Slovennia was third (14,350).
It is her second title in almost a week.
Earlier, Thanh topped the same event at the Artistic&Rhythmic Gymnastics World Top Competition in Incheon , South Korea.
The World Cup winner will later take part in the all-round and parallel events.
In Osijek , Thanh's teammate Pham Phuoc Hung claimed a silver medal in the men's parallel category. Hung won 15,625 points to bag the second place, a big jump compared to the sixth position he scored during qualification rounds.
Slovenia 's Mitja Petkosek who took 15,725 points won a gold medal while Oleg Stepko of Azerbaijan came third with 15,325.
Another Vietnamese, Dang Nam, will later compete in the men's rings.
The Vietnam team is participating in the tournament with five athletes. They are also members of the squad that will compete at the coming 18th Asian Games in Incheon , South Korea , in September.-VNA
