Thai came second in the men's junior road race with a time of 2hr 50.23min, just a hair behind Dinmukhammed Ulysbayev of Kazakhstan. The host's Hanada Kiyomasa finished third and claimed the bronze.
Another Vietnamese Tran Duc Tien placed fifth.
The event featured more than 40 athletes racing a 107km stage.
Thai and Ulysbayev broke away from the peloton at the 70km mark, opening up a one-minute gap with the chasing group.
Two days ago Thai also came second in the time trial event but his result was not recognised as judges said he took a short cut & crossed the centre white line.
Vietnam's Pham Le Xuan Hong competed in the women's junior road race. She did not finish.
Cyclists are due to participate in the 36th Asian Cycling Championships's women's 167km road race event at the same venue on January 23.
Representing Vietnam are Southeast Asian Games winner Nguyen Thi That and Phan Thi Lieu.
Meanwhile Huynh Thanh Tung will compete in the men's U23 119km road race event.
