The Vietnamese player won in the under-10 open category with 10 points after 10 rounds.
Khoi, who is seeded No 13 at the tournament, won all his matches.
Khoi is the sixth winner from Vietnam at the annual tournament for juniors.
Earlier he won a doubles at the Asian Chess Championships with two golds in the blitz and rapid chess events.
Vietnam has taken part in the Maribor event with 12 masters in different age groups.-VNA