Six weeks ago, in a pugnacious proud toast to a new Soviet-Vietnamese friendship treaty that is aimed squarely at China, Leonid Brezhnev declared the pact to be "a political reality and whether they want it or not, they will have to reckon with this reality."

Today, a Chinese source permitted himself a slight smile as he recalled Brezhnev's word and savored the unpleasant new political reality confronting the Kremlin: The announcement yesterday that the United States and China would establish normal diplomatic relations on Jan. 1.

The diplomatic breakthrough is the most spectacular achievement of the new Chinese leadership since Peking began its concerted efforts this year to end decades of isolation and challenge the Soviets around the world while at the same time seeking aid from the capitalists to medernize its backward economy.

The Soviet leader's toast of Nov. 3 netly summarizes the Kremlin's predicament following the Peking-Washington announcement: It is something they will have to learn to live with and it is going to be lesse than pleasant learning.

The news comes at a delicate moment in the Soviet-American relationship, which has been on the upswing after more than a year of friction. The two nations are nearing completion of a Strategic Arms Limitation Treaty which could lead to a Carter-Brezhenv summit meeting as early as next month in Washington.

Meanwhile, Soviet apprehensions about the Chinese have increased to a feverish level in recent months, triggered by Peking's diplomatic successes in such places as Japan, Romania, and Yugosalvia, where the Soviets have long had important interests in minimizing China's access and influence.

The Kremlin's concern over Peking's successful breakout has been immeasurably sharpened by the seeming readiness of France and Britain to sell China advanced weapons, such as the Harrier jet fighter and antitank missiles, and to find financial credits for other major deals which the Soviets themselves would like to have.

Diplomats here were stunned by the news, saying they did not expect the White House to arrange for resumption of full relations until sometime after the Brezhnev-Carter summit. The Soviets have repeatedly stated that SALT is too important to be affected by any other bilateral concerns.

"The Carter timing is going to put taht to the test," one Western diplomat observed.

He said the first sign of Soviet reaction may come at the Geneva talks next week between Secretary of State Cyrus Vance and Soviet Foreign Minister Andrei Gromyko. It had been thought by several well-informed Western sources here that the Geneva talks would wrap up the last, stubborn issues on SALT II and clear the way for a summit meeting.

Beyond a brief news item by Tass, the government press agency, there has been no public Soviet comment here. It is customary in such situations for the leadership to carefully work over a response before making its views known.

Some diplomats speculated that Peking-Washington normalization could act as a spur to the Soviets to reach agreement on SALT more quickly as a means of cementing the special relationship they say hinges on the arms pact, which is described by Soviet officials as the heart of detente.

In a range of interviews with European and Asian diplomats today, all expressed virtual unanimity that the Soviets will find it difficult to raise initial objections over normalization.

"This is something they have criticized the U.S. for not having for perhaps 20 years," one senior diplomat said, "Of course, their view has changed a little from former days, but I should think they will find it difficult to complain about. The Soviets traditionally have favored normalization of relations between states.

At the same time, these sources say they believe the Soviets would view with alarm and U.S. effort to begin major arms or technology transfers to China.

"That's the bottom line for them," one sources said. "It is something they have made quite clear on many occasions."

Indeed, the Soviet attitude toward Peking has hardened through 1978 in the face of continued Chinese successes in diplomacy and at the negotiating table. The principal cause of Kremlin anger and alarm this fall is the Sino-Japanese treaty peace and friendship treaty, which contains an "anti hegemony" clause the Soviets bitterly inpret as being directed at them to blunt their legitimate attempts at influence in Asia.

The Peking-Washington Communique has a similar antihegemony clause, pledging the two nations not to seek dominant influence in Asia or anywhere else and to oppose similar attempts by any other nations. The Kremlin seems certain to view the clause as aimed at them and this may be where the strongest Soviet reaction will come.

What form such a response could take is unclear. The Soviets in recent months have issued a variety of warnings to the West in general, and to the United States in particular against trying to play the "China card." Initially, the Kremlin ire was directed at Carter's national security adviser Zbigniew Brzezinski, but then it broadened into more general warnings. In a June speech in Minsk, Brezhnev asserted that the White House was pursuing "a short-sighted and dangerous policy" in its attempts to improve and strengthen ties to China. "It's architects may bitterly regret it," he declared.

In early November, Georgi Arbatov, a senior adviser to the leadership U.S. relations, in an interview with a Western journalist asserted that while the Soviet Union is not "scared by China," and favors normalization, "the situation would look different to us "if China established itsel as "some sort of military ally of the West."

"If such an axis is built on an anti-Soviet basis, then there is no place for detente," he said.

As if to test this perspective, in late November Brezhnev sent a private letter to British Prime Minister James Callaghan threatening serious consequences for Anglo-Soviet relations of the British went ahead with the sale of Harrier jets to China. The Brezhnev move was viewed here as a tactic which, if successful against the British, would be used in dealing with other countries to block arms sales to Peking.

Last week in Sofia, a top Kremlin ideologist, Boris Ponomaryov, attakc China and asserted that Western arms sales to China could result in a new world war. Speaking at an ideological conference, he said the Warsaw Pact was "forced to make corresponding measures to strengthen their defense capability." It was the first time the leadership publicly linked the issue of China to a dispute with Romanian President Nicolae Ceausescu last month over his opposition to increased defense spending by the Warsaw Pact countries.

Exactly what form the Soviet threats would take if Moscow felt encircled by a strengthened NATO on the West and a military improved China on the east is impossible to say. Several sources suggested, however, that if the Kremlin begins to feel pressured about "encirclement," it might attempt a breakout of its own, in the form of trouble over Berlin, or new adventurism with its Cuban allies in Africa, or pressuring the Firms into joint military maneuvers along a sensitive northern front for NATO.

But that kind of long-range doomsaying seemed out of proportion of Friday's announcement.

"Normalization shouldn't have any serious, abrasive impact on U.S. Soviet relations so long as there are no arms sales or technology transfers," one source summarized. But he then cautioned that the Gromyko-Vance talks in Geneva beginning Thursday may tell much about the Kremlin's view of the new Peking-Washington detente.