 After pacifying central Vietnam , Duyệt was reappointed to be the viceroy of southern Vietnam and Cambodia in 1820 by Emperor Minh Mạng , Gia Long 's successor . The emperor further enhanced Duyệt 's powers by allowing him the power to oversee all foreign trade coming into his area , and collect taxes in Cambodia and on imports and exports at his discretion . This gave Duyệt control over the vast economic resources in the fertile agricultural and lumber @-@ rich region and most importantly , land development . The southern area had only been populated by ethnic Vietnamese in recent centuries and immigration , clearing and development of land was rapid . 
