Cambodian Prime Minister Hun Sen said Friday all of Vietnam's advisers and nearly three-quarters of its troops have left his country, but he said his government can defend itself against the Khmer Rouge and other guerrillas. Noting that all Vietnamese troops are to be out by 1990, Hun Sen said, "We still have two years and we must race quickly to get ready." Hun Sen spoke to about 150 foreign correspondents in Phnom Penh. The partial Vietnamese pullout, said to involve 18,000 troops, began Thursday and will continue until Wednesday.