IN ancient times the emperors of China required foreign envoys to face north when paying tribute and performing the obligatory kowtow.But when the current autocrats of Beijing accepted the obeisances from Vietnam's leaders during a recent visit, they pointed them toward the most venerable point in the communist world: China's thriving, entrepreneurial south. China's effort to showcase its special economic zones in the south reflects how much Sino-Vietnam relations have changed since the two countries were last on speaking terms 13 years ago. It also reflects how China regards its role as the lone protector of Marxism. There were no paeans to Marx during the six-day visit ending Nov. 10 by Communist Party leader Do Muoi and Prime Minister Vo Van Kiet. Instead, the Vietnamese visitors journeyed to Shenzhen in Guangdong Province to see first-hand the wealth from market economics and eased social regimentation. In Shenzhen, a vibrant enclave of entrepreneurship next to Hong Kong, the leaders toured an industrial exhibition hall, a factory run by Hitachi Company, and the International Trade Center. Mr. Muoi invited businessmen and executives at the center to open branch offices in Hanoi. The Vietnamese were not the first communist visitors targeted for conversion to Chinese-style reform. China's leadership guided North Korean leader Kim Il Sung on a similar junket to Jiangsu Province, another of China's fastest-growing areas, in October. The Jiangsu tour included a visit to a foreign joint venture, the sort of arrangement Beijing believes Pyongyang must adopt if it hopes to prevent the collapse of the North Korean economy. It is an open secret that China has encouraged Mr. Kim to soften his hostility to capitalist countries and market-oriented reform. Beijing believes its emphasis on economic reform in its diplomacy can help ruling communist parties in other countries survive the worldwide rejection of Marxism, Western diplomats say. China still colors the visits of socialist dignitaries with Marxist dogma. In private meetings with these envoys, Beijing condemns "foreign forces" for promoting "peaceful evolution" in China, a code word used to describe a Western conspiracy to subvert Marxism in favor of capitalism and democracy. But the sudden downfall of communist regimes in the former Warsaw Pact showed China the danger of basing its diplomacy toward communist countries on relations between parties rather than between states, the diplomats say. For Vietnam, the instruction on economic reform is just another humbling step signaling its diplomatic defeat to its more powerful neighbor. Vietnam also caved in and met Beijing's chief condition for normalized relations by condoning the Cambodian peace accord in October. Vietnam is desperate for trade and aid now that the Soviets have cut such benefits. It also hopes that improved relations with China will help link it to a lucrative trade network between its own ethnic Chinese and those abroad. Improved relations with Vietnam will help China develop its impoverished areas in southwest China. The two countries signed a trade agreement during the visit and China plans to reopen two railway lines running from its poor, mountainous interior through Vietnam to the Gulf of Tonkin. Still, Beijing has said it does not want to rush the rapprochement. By seeming hesitant, it aims to extract further concessions from Hanoi on border disputes, nearly $187 million in debt, and conflicting claims to islands in the South China Sea.