Basetsana were hoping to recover from their 3-0 defeat to Thailand in their opening game of the event, but failed to get their campaign back on track in Xiamen.
On the other hand, the Vietnamese, who crumbled 4-0 against hosts China, outclassed the Africans to claim their first win of the tournament.
The opener was scored in the 13th minute before Vietnam doubled their lead in the 26th minute of the game at the Haicang Sport Centre Stadium.
Sheryl Botes' side let in the third in the 54th minute and they were condemned to their second defeat on the bounce in the competition.
The defeat means South Africa are on bottom of the table with no point after two games, with Friday's game against hosts China their final one of the event.
In the other match, leaders China were held to a 0-0 draw by second-placed Thailand.
Thailand and Vietnam will face off in Friday's early game.