President Valery Giscard d'Estaing voiced concern to U.S. Secretary of State Cyrus R. Vance today about an evident softening of American opposition to the Socialist-Communist alliance that is seeking to take control of France politically, informed sources reported.

Giscard, whose centrist and conservative supporters were drubbed by the Socialists and Communists in municipal elections last month, indirectly but unmistakeably suggested that recent American contacts with the two leftists groups are undercrutting his position, according to the sources.

Stopping over here immediately before returning to Washington, Vance briefed the French leader on the collapse of nuclear weapons limitation talks in Moscow earlier this week. Whille concerned about the impact of new difficulties on detente, French officials reportedly view the failure in Moscow as a "temporary" setback.

The discussion of official American attitudes toward the increasingly strong French left was seen here as another sign of Giscard's sensitivity to the growing acceptance the Communists are gaining through their alliance with the larger, less doctrinaire Socialist Party.

The Carter administration has not carried over the aggressively worded warnings frequently sounded by Henry Kissinger during the Nixon and Ford administrations. Kissinger stressed that the United States could not have close cooperation with a Western European government that included Communists.

In meetings with two senior officials of the Socialist Party in February in Washington, Vance put a significantly different emphasis on what Washington's reaction would be a victory by the Socialist-Communist alliance in the legislative elections scheduled 11 months from now, according to the Socialists and other informed sources.

Vance repeated the Kissinger formulation that Eurpean voters are free to choose their own governments, but Washington would reassess its view toward governments with Communist participation, according to American and French Socialist versions of the conversations.

While in office Kissinger strongly accented the probablility of a hostile American reaction. Last year he instructed American diplomats here to go out and see Socialist leaders to hammer that point home.

Vance, however, put his emphasis on the assurances that the United States would not interfere in internal French affairs, if the Socialists and Communists beat Giscards right-center coalition in the legislative elections.

Reports of the conversations between Vance and Socialist leaders Michel Rocard and Jean-Pierre Cot leaked into the French press last month at about the time it became known here that two American diplomats had visited Communist Party Palitburo member Jean Kanapa for a discussion in his office.

Kapana, one of the most important figures in the party, is considered by many French analysts to be one of the more "Stalinist officials in a party that now consistently claims it is following a more liberal, politically tolerant "Euro-Communist" program.

The talk with Kanapa broke little new ground, according to informed sources, but it has created a stir in the French press, which has interpreted the meeting as an important shift in American policy.

Embassy sources and a State Department spokesman in Washington sought to minimize the significance of the meeting, which they said was part of a continuing process of talks between lower-level U.S. diplomats and local Communists.

Under guidelines established at the embassy, only the ambassador and his top political aides refuse to have any formalized contacts with the Communists.

But information about past lower level meetings here and in Rome with Italian Communist officials was kept secret during Kissingers reign, and mention of the meeting with Kanapa has had the impact of suggesting a policy change that Giscard reportedly indicated today could amount to interference in French internal affairs.

Vance reportedly assured the French president that the press had taken the American contacts with the left out of context, and had been inaccurate on some points.

The Moscow negotiations and the French political question took up relatively little time in the meeting, which concentrated on Africa. France is deeply concerned about the fate of President Mobutu Sese Sekos regime in Zaire and the future of Djibouti, which is to become independent in May.

The French president also sought once again to press home the probability of a strong hostile reaction by French public opinion if the Concorde supersonic jetliner is not granted landing rights in New York. Walked with Vance from the Elysee Palace meeting room, Giscard asked him, "Are you going back on Concorde?"