Hai netted the ball from Negerian-born Dinh Hoang Max’s cross in the last minutes of the game, stunning substitute goalkeeper Pavel Kovac.
The friendly game marked the first-ever appearance of Ukraine-born goal keeper Dinh Hoang La and Dinh Hoang Max, who took Vietnamese citizenship, in the national team jersey.
Olympiakos took their first chance when Kotantinos Mendrinos launched a 30m kick that hit the bar, threatening the keeper.
Vietnamese trio, Pham Thanh Luong, Nguyen Viet Thang and Le Cong Vinh, worked well together, creating opportunities to get closer to wooden box. But they failed to beat goalie Antonios Nikopolidis, who won the European Championship in 2004, in the first half.
After the break, striker Derbyshire had three chances to face Vietnam’s keeper in the first minutes of the second half, but Ukrainian goalie Dinh Hoang La made perfect saves.
In the 83rd minute, left-wing defender Le Tan Tai broke through into penalty area to face Olympiakos’s keeper Kovac, but his shot bounced off the goalie. Max came in time to take a shot from rebound, but it wide out the bar.
Two minutes later, Quang Hai made a break through to face Kovac, but his shot just skimmed the post, missing a goal. The chance came up in the 88th minute when he trapped a cross from Max to turn the ball to the back of the net.
The Greek champions also missed a clear chance when Derbyshire tapped his teammate’s cross at the goal mouth, but it went wide.