The 46 asylum seekers were aboard a small vessel intercepted off Australia's remote west coast last year and were returned to Vietnam as a result of negotiations between the two countries.
The defendants, two men and two women, were crossing the border illegally for the first time in July 2015 and their 42 Vietnamese companions were relatives and acquaintances, lawyer Vo An Don told Reuters.
Vietnam's foreign ministry did not immediately respond to Reuters' request for comment.