China's relationship with Vietnam was as close as that between ''lips and teeth,'' to quote the oft-used phrase of more than 10 years ago. Friendship Pass was its symbol.
Today, the relationship is one of ''rifle against rifle, cannon against cannon,'' said a Chinese official briefing foreign correspondents touring the Sino-Vietnamese border.
Youyi-Guan, Friendship Gate, still stands. But the road that once went through it and down a gentle incline to the actual border a few hundred yards beyond now carries no traffic. What besides mines and gun emplacements there may be on the Vietnamese side of the border, the Chinese say they do not know. Once or twice a year, an exchange of prisoners captured by one side or the other in the constant exchanges of fire and border incursions takes place at the wire barrier between China and Vietnam. That is about all.
From Friendship Gate, one can see across the green of hills and valleys streaked with granite gray to a barely discernible cluster of huts about a mile and a half away. That, we are told, is the nearest Vietnamese village.
It is not as tense a border as one had been led to expect in Peking, where Xinhua agency dispatches told of a school or a hospital having been ''destroyed'' by Vietnamese shells.
At remote Pingmeng, about 120 miles northwest of Friendship Pass, in a wildly beautiful stretch of mountain valleys, the school this correspondent visited had received a total of four 82-mm mortar rounds, one landing in the front yard, another in the back, and two in classrooms. The attack took place in April, at 5 p.m. The principal of the school said no children were in school at the time of the attack.
We were also shown a restaurant and a hotel that had been hit but that still functioned, and a hospital that had been evacuated as being too near the border. A 105-mm shell had come through the roof and pierced the floor of a second-story room, then passed through the wall of a ground floor room to fall, unex-ploded, into the courtyard, a hospital official said. The hospital had already been evacuated by then. Later we were shown an unexploded 105-mm shell: It bore American markings and appeared to be part of the huge stock of arms Hanoi captured at the end of the Vietnam war.
At Pingmeng we met a woman who had been wounded by a sniper's bullet while riding on a small tractor. The tractor's driver had been permanently disabled and was now working as a clerk in an office. This woman, a member of the Zhuang minority, as are most of Pingmeng's inhabitants, had relatives across the border , where the Zhuang are known as Nong. She had not seen these relatives for eight years, she said.
During China's ''counteroffensive in self-defense'' against Vietnam in February 1979, the inhabitants of Pingmeng were all evacuated to towns further from the border. Only a year after the border war were they allowed to return to Pingmeng. More than 130 former inhabitants still have not returned to the village. Among the returnees, more than 100 still live in flimsy temporary shacks because their fields used to be on the Vietnamese side of the border and are therefore inaccessible to them today. These people have had to find work on other people's fields, or in nonagricultural pursuits, in order to survive.
The Vietnamese are also still forcing ethnic Chinese to leave their country, according to Zhang Zhongliang, deputy chief of Gaungxi Province's foreign relations bureau. Last year, Mr. Zhang said, a total of 1,097 refugees from Vietnam arrived in Guangxi Province: about 800 by sea, the rest by land.
The refugees are housed in one of two reception centers supported by the United Nations High Commissioner for Refugees. They are given work, either on a state farm built expressly for refugees from Vietnam, or in appropriate administrative or industrial jobs. Those with relatives in the United States, Canada, or other foreign countries often apply for emigration.
Chinese officials say that if the country to which the refugees make application is willing to accept them, no effort is made by Peking to prevent them from leaving the country. From this viewpoint, the land route to China may be a safer way for refugees to get out of Vietnam than the hazardous voyage by boat through pirate-infested waters to the Philippines or Malaysia.
Did Chinese officials on the spot expect to see Sino-Vietnamese relations return to the old friendship and cross-border contacts?
''It is difficult to foresee,'' an official said. ''At present, our policy is , if you hit me, I hit you. If you open fire, I open fire.''
He then, however, tied a relaxation of tensions on the Sino-Vietnamese border directly to Vietnamese actions in Kampuchea. It was the Vietnamese invasion of Kampuchea in December 1978 that in turn sparked China's ''counteroffensive in self-defense'' two months later.
''If Vietnam abandons its anti-Chinese acts along the border,'' the official said, ''if it withdraws its troops from Kampuchea, if it shows itself ready for friendly talks on the border question, then the tense situation along the Sino-Vietnamese border can be solved.'' Would there be another ''counteroffensive in self-defense'' if Vietnam did not take this course?
''That would be difficult to say,'' the official replied. ''As long as Vietnam does not cease its provocations, we will retain the right to make a counterattack.''