A Vietnamese court convicted 18 Vietnamese exiles of trying to overthrow the Communist government in Hanoi and imposed sentences ranging from three years to life imprisonment. Vietnam said the 18 were among a small army that infiltrated into Laos from Thailand last July en route to commit sabotage in Vietnam. They were captured after a six-week running battle with Vietnamese and Laotian troops. A judge at the trial in Ho Chi Minh City said the group was formed in 1980 in San Jose, Calif., by former South Vietnamese military officers. He said they then established bases in northeastern Thailand, where they received support from Thai and U.S. intelligence agencies.