President Carter said yesterday that Secretary of State Cyrus R. Vance will leave for Moscow today "with high hopes" for progress in negotiations between the United States and the Soviet Union and with a new American proposal for "substantial reductions" in the number of nuclear weapons.

Prefacing his remarks with a reiteration of his faith in the value of discussing such matters openly, the President set forth in broad terms the major policy objectives that Vance and his team will take to Moscow.

He also suggested that he is close to the point at which he would support normalization of relations between the United States and Vietnam, and said he is convinced that Soviet officials do not want the controversy over human rights to impede progress in nuclear arms talks.

Carter began his fourth press conference as President by laying out the main issues to be discussed at the Moscow talks. He did not cite the actual arms reduction that the Vance mission will seek, but said "the central focal point will be arms limitations and actual reductions for a change."

Should that initiative fail, the President said, the United States' "fall-back" position will be to seek the ratification of the arms levels contained in the 1974 Vladivostok accord between the United States and the Soviet Union.

That accord allows each nation up to 2,400 strategic nuclear weapons, a level that Carter characterized yesterday as "just ground rules for intensified competition and a continued massive arms growth in nuclear weapons."

In addition to nuclear arms reductions, the President said, the Vance mission to Moscow will seek agreements to:

Limit arms sales by the United States and the Soviet Union to other nations. The United States, he said, has been "working with our allies to cut down this traffic, and we hope to get the Soviet Union to agree with us on constraint."

Control the testing of nuclear devices, including non-military devices, with the U. S. objective being "to eliminate these tests altogether if the Soviets will agree."

Achieve "mutual and balanced force reductions" in Europe. When he returns, Vance will "report on the attitude of the Soviet Union leaders concerning the European theater," Carter said.

The President said Vance will also seek to make progress toward "demilitarizing the Indian Ocean," removing from the African continent "outside interference which might contribute to warfare in the countries involved" and to lay the groundwork for Soviet cooperation at a Geneva Conference on the Middle East planned for later this year.

Carter's opening statement was the latest and most clear-cut example of what has become a hallmark of his presidency - the relatively open discussion of sensitive foreign policy issues. In effect, he used the press conference to notify the world of U.S. objectives for the Moscow meeting before the Soviets respond to the American proposals in the negotiations.

The President prefaced his remarks by noting that there has been some criticism of his freewheeling style in discussing foreign policy. But arguing that it is "very important that the strength of the presidency itself be recognized as deriving from the people of this nation," he said he intends to continue to inform the nation of "what is going on" and what options he is pursuing.

Carter praised Soviet leaders for their "very cooperative" attitude leading up to the Moscow talks, and said he is convinced they will set aside differences over the human rights issue in the interest of "substantial progress" toward arms limitations and other goals.

In a speech to a national trade union congress Monday, Soviet Communist Party leader Leonid I. Brezhnev attacked Carter's support for Soviet dissidents as "interference in our internal affairs," and warned that "normal development of relations on such a basis is, of course, unthinkable."

Asked about this yesterday, the President said that despite the disagreement over human rights there is "nothing that I have heard directly or indirectly from Mrs. Brezhnev that would indicate that he is not very eager to see substantial progress made in arms limitations."

He said he does not consider his vocal support for human rights around the globe to be an intrusion in Soviet affairs and that Brezhnev's cooperation in arranging the Moscow talks "is adequate proof that he has not broken off relationships in any way and that he has hopes that the talks will be productive."

On Vietnam, Carter came as close as he ever has to supporting normalization of relations with the Southeast Asia country. With the return this week of the U.S. mission on American servicemen missing in action in Southeast Asia, the President said he believes the Vietnamese have done "about all they can do" to help in the search for the missing.

The President said that if negotiations in Paris between the United States and Vietnam show the Vietnamese to be acting in "good faith" he will "aggressively move to admit Vietnam to the United Nations and also to normalize relations with them."

But Carter also took a hard line on the issue of U.S. reparations to Vietnam - a point the Vietnamese have insisted would have to precede an accounting of the MIAs.

"I don't feel that we ought to apologize or castigate ourselves or to assume the status of culpability," he said. "I am willing to face the future without reference to the past. That is what the Vietnamese leaders have proposed."

Asked about U.S. involvement in Zaire, the President said the United States has no military obligation to the country but is cooperating with the Belgian and French governments in an effort to contain the armed conflict in Zaire.

Carter also noted that the United States has "substantial commercial investments" in the African nation. He did not get specific.

U.S. officials estimate direct U.S. investment in Zaire at about $200 million, largely in an offshore oil exploration project and an inactive copper producing venture in Shaba province.

U.S. government-guaranteed loans to Zaire are estimated at about $500 million, most of this in a proposed power transmission system from Kinshasa to Shaba province. Private, unguaranteed U.S. bank loans are estimated at less than $200 million.