Secretary of State Cyrus R. Vance said yesterday that he will take United States "proposals" and "suggestions" to Middle East leaders next week in an effort to break the logjam blocking an Arab-Israeli peace conference.

Preparing to depart Sunday night on a 12-day journey to the region, Vance told reporters that the U.S. proposals involve both the procedures and the substance of a Geneva conference. He would not disuss details of the proposals, saying that they will be submitted to Arab and Israeli leaders on a confidential basis.

President Carter has repeatedly said that the United States does not intend to impose its own plan or solution on the parties to the Middle East dispute, but he has not ruled out American proposals, suggestions and ideas put forth in its role as an intermediary.

Vance's remarks and those of other U.S. officials indicate that he plans to be an active rather than a passive mediator, seeking to narrow the gap between Arabs and Jews by suggesting which of their positions are and which are not within the range of possible agreement and suggesting compromise solutions that might be mutually acceptable.

Give the wide gap between Arab and Israeli positions, the long history of the bitter dispute and the fears and passions it has engendered, officials experienced in Middle East affairs are not optimistic about the chances for quick success. Despite a claim that there is no difference of view, Vance himself seemednotably less optimistic in his remarks yesterday than Carter has been on several recent occasions.

"How much we can accomplish during this trip remains to be seen," Vance said. He said he may not know at the end of the trip whether a Geneva conference will be possible this fall. If this is the case, his plan is to continue the discussions with Arab and Israeli foreign ministers when they come to New York for the United Nations General Assembly session in September.

No shuttle diplomacy is on Vance's schedule for the forthcoming trip, which calls for stops in Egypt, Lebanon, Syria, Jordan, Saudi Arabia and Israel, in that order. Before leaving Washington, he said, he plans to discuss the U.S. proposals with officials of the Soviet Union, which cochairs with the United States the Geneva peace conference on the Middle East.

Vance said he has been keeping "closely in touch" with the Soviets in recurrent Middle East discussions. The Soviets, he said, have indicated a willingness to "use their influence" with some of the parties to encourage flexibility.

Vance also announced at his news conference that he and Soviet Foreign Minister Andrei Gromyko will resume the strategic arms limitation talks in Vienna Sept. 7-9. He said disarmament negotiators of the two nations in Geneva have been assigned to work on some of the problems that were not resolved in the May meeting between himself and Gromyko.

The Secretary of State also defended U.S. plans to provide arms to Somalia and Sudan, both of which have been Soviet arms recipients, saying "This in no way is to be construed as an attempt to enter into an arms race with the Soviet Union." He said the arms for Somalia would be "defensive" in nature and that they undoubtedly would be supplied only on the condition that they would not be used in the Somali-backed insurgency in neighboring Ethiopia.

The major procedural roadlock to a new Geneva conference on the Middle East is representation there of the Palestine Liberation Organization (PLO), which has been designated by the Arab states as the sole legitimate representative of the Palestinian people. Israeli Prime Minister Menahem Begin says he will object to the presence at Geneva of any "known member" of the PLO, while Arab leaders in the past have insistedon a PLO role.

Vance said: "I do not expect that there will be any meeting with the PLO during this trip." He said there has yet been "no suggestion" by the Palestinians that they are willing to do the things Carter outlined at his press conference Thursday - in essence, to accept the permanent existence of Israel. In the absence of such a change, the United States feels "constrained" by the 1975 agreement with Israel not to deal with the PLO, he said.

Meanwhile, Assistant Secretary of State Alfred L. Atherton briefed the Senate Foreign Relations Committee in executive session yesterday about the administration's plan to provide about $100 million in military sales credits over the next three years to equip a 3,000-man Lebanese Army force with mortars, machine guns and other light infantry equipment.

Reports from Israel have said the Israeli government does not object to such arms for the Lebanese army, which would be expected to assert control in areas of Lebanon now dominated by Syria or the PLO. However, some U.S. lawmakers have interposed objections to the plan.

The administration has not formally presented the Lebanese arms plan to Congress. Vance is expected to discuss the plan with Lebanese President Elias Sarkis during his Mideast trip.

Vance said yesterday that he will discuss a plan to station United Nations observers in southern Lebanon. Begin recently endorsed the idea.