The United States and the Soviet Union have agreed to a round of concrete negotiations aimed at creating common guidelines on the supply of conventional weapons to Latin America.

If agreement can be acheived at the talks scheduled for December in Mexico City, it will be the first collaboration by the two leading arms suppliers to hold down the convention arms race in any region. Such a superpower accord would have promise of extension to several other areas and inplications for the arms traffic throughout the world.

In another decision confirmed during the visit here of Secretary of State Cyrus R. Vance, the two countries have agreed to a new round of negotiations on banning antisetillite weapons and thus heading off the growing possibility of an arms race in space. The new talks also will take place in Latin America, this time in Venezuela.

According to informed sources, these decisions and renewed resolve in several other arms control questions were the substance behind a little-noticed part of the Soviet announcement Monday night after Vance's meeting in the Kremlin with Soviet's leader Leonid Brezhnev.

Although the announcement, like the meeting itself, concentrated on strategic nuclear arms, it also said that both sides intend "to facilitate progress in negotiations on other questions of arms limitation and disarmament with the purpose of attaining concrete accords in the matter."

The decision to zero in on Latin America as a field for control of conventional arms is the most novel of the recent Soviet-American moves. The two nations held preliminary discussions last December in Washington and continued them in Helsinki last May. Those talks covered the general theory and principles of a policy of arms restraint. The Mexico City talks will be an attempt to discover if the principles can be constructivelu applied in practice.

As proposed by Soviet Foreign Minister Andrei Gromyko in an address to the United Nations last month, the Soviet idea is to base the control of weapons sales on "international legal criteria" which take into account "the legitimate interests of people fighting against agression, for their freedom and independence."

According to persons familiar with the Soviet position, Moscow insists on the right to supply weapons in "wars of liberation" and "just wars," especially those endorsed by the United Nations.

This would permit the continued supply of Soviet arms to the black nationalist forces around Rhodesia and South Africa, Palestine groups in the Middle East and, on grounds of defense against agressive neighbors, the continued Soviet arms supply to Ethopia and Angola.

The Soviets also propose that the principles ban the supply of arms to nations refusing to agree to arms control pacts - an apparent reference to their arch-rival, China.

Soviet ideas and arms supply policies clash with those of the United States in many cases. However, Latin America - far from Soviet shores and perpheral to Soviet interests - is considered the most promising area for negotiating common policies.

The United States has long sought to restrain the sale of major weapons in Latin America, refusing to sell high-performance aircraft and other sophisticated arms.

The Soviets have provided major weapons to Cuba, its only ally in the area, and in 1976, after the United States balked at providing F5 fighter-bombers to Peru on terms acceptable to that country, the Peruvians went to Moscow and bought 36 high-performance SU 22S on bargain terms. The deal touched off concern about a Soviet-American arms race and Soviet military penetration in the region. Peru subsequently bought several other Soviet items.

When the United States reduced its military aid to Argentina early last year on human rights grounds, and Buenos Aires then refused any further aid on grounds of national pride, the Argentine military was reported to be investigating purchases from Moscow.

France, Britain, and several other countries also supply some major arms to the Latin nations. Thus any Soviet-American agreement, to be fully effective, would have to be accepted by the Europeans as well.

While U.S. officials have said that many difficulties could impede collaboration in the control of conventional arms sales, they have also expressed encouragement about the willingness of Moscow to consider the matter.

The antisatellite weapons negotiations, which began last June in Helsinki, are in an early stage. Venezuela, which is on friendly terms with both superpowers, evidently was selected as the site of the next negotiating round as further sign of potential U.S.-Soviet accord in the region.

Space weapons talks are important because they involve the use of satellites for surveillance. "It's the heart of the SALT vertification system," said an official, referring to the strategic arms limitation treaty now under negotiation. "SALT agreements are meaningless without the satellite means of verification."

Among the other arms control matters touched on during Vance's sessions here were:

Future negotiations on medium-range nuclear weapons placed on either side of the East-West line in Europe. This is expected to be an important topic in SALT III, which would follow signing and ratification of the SALT II accord currently being worked out.

The separate East-West trade talks in Vienna on tre. U.S. and Soviet officials are seeking ways to bridge a longstanding disagreement over the number of troops already in place.

The comprehensive nuclear test-ban treaty now in a late stage of negotiation at Geneva. The only major problem remaining, according to the sources, is the number and placement of seismic stations to be located on U.S. and Soviet soil to monitor compliance with the ban on nuclear tests. Now that the United States had reduced the proposed duration of the treaty from five years to three years, the Soviets are insisting that fewer seismic stations are needed to monitor complaince.

(In Washington, the Associated Press reported that President Carter, unwilling to give up hopes of completing SALT by the end of the year, is planning to send Vance to Geneva next month for a ninth round of talks with Gromyko.)