Withdrawal of Israeli, Palestinian and Syrian troops from Lebanon would create a vacuum that might have to be filled by sending more Marines to the volatile country as part of a larger multinational peace-keeping force, Defense Secretary Caspar W. Weinberger said yesterday.

Although he stressed that "there is no specific number and no specific time" for expanding the present 4,000-man multinational force, Weinberger's remarks emphasized the likelihood of a long stay for the Marines in Lebanon.

The blueprint he sketched yesterday for bringing peace to Lebanon also contrasted sharply with the one drawn by his new counterpart in Israel, Moshe Arens, the Israeli ambassador here who has been named defense minister.

"As we get the agreement we hope we will get from the Israelis and the Syrians and PLO Palestine Liberation Organization representatives, and the forces pull back, this opens up a wider and wider range of territory that needs to be covered," Weinberger said at a Pentagon news conference.

"And, as you do that, you need additional forces of some kind to fill that gap and continue to provide that degree of confidence to both sides as they withdraw that one of them isn't going to turn and fire at the other or some mistake happen," Weinberger said. "So that's why it may be necessary to expand to some extent" the multinational force.

"It's not a permanent thing at all," Weinberger continued. "The permanent thing is to get Lebanon back as a strong, sovereign nation with its own armed forces in sufficient degree of equipment and training and readiness so that you don't need this additional force. It's very undesirable from my point of view to have a permanent foreign force presence in Lebanon, just as it would be in any sovereign country."

To reassure Israel about the Lebanese border area, Weinberger said, "We're trying to work out" putting "a number of Lebanese armed forces units" in southern Lebanon for training, which "in itself might very well exclude or preclude the presence of any unfriendly forces in that area for which Israel could justly complain."

But at a Washington Post luncheon Tuesday, Arens said: "I think without any help from Israel it will probably be difficult" for Lebanon to provide for its own security within the next five years.

"Just what the United States has in mind in guaranteeing the security of Israel's northern border I really don't know, and I think it's something we want to talk about," Arens said. "We've had a long tradition, and I don't think we're about to go from it, that the only guarantee for Israel's security is the Israel Defense Forces."

Weinberger's comments came amid new reports that President Reagan is chafing at Israel's slowness in agreeing to a withdrawal from Lebanon. At a Wednesday breakfast meeting with reporters, Reagan also spoke about the need for a Mideast peace solution "providing something in the nature of a homeland" for the Palestinians.

The term "homeland" has always angered Israelis, who regard it as a code word for an independent Palestinian state. However, administration officials stressed yesterday that Reagan intended no change in the position he spelled out in his Sept. 1 Middle East peace initiative.

Calling in that speech for "a just solution . . . to the legitimate rights of the Palestinians," Reagan said it could be achieved by giving the Palestinian inhabitants of the West Bank and Gaza Strip "self-government in association with Jordan." He rejected in his initiative an independent Palestinian state.

Immediately after referring to "a homeland" in his remarks Wednesday, Reagan added, "On the other hand, no one has ever advocated creating a nation."

Following Reagan's remark Wednesday, Arens asserted that "Israel's position is that a Palestinian homeland and state exists--Jordan." However, Secretary of State George P. Shultz, speaking yesterday in Atlanta, said Israel "can't pass off the Palestinian issue" by declaring Jordan a Palestinian state. He added that the solution proposed in Reagan's initiative addresses "the legitimate rights and aspirations of the Palestinian people" with "ingenuity and creativity."

Administration officials acknowledged yesterday that Reagan wants to keep pressure on Israel over Lebanon. But, these sources added, the president may be less impatient now than he was before Israeli Prime Minister Menachem Begin was forced to replace Ariel Sharon with Arens as defense minister.

In the view of these officials, Arens can be expected to bargain hard for Israel's goals. However, unlike Sharon, who was regarded here as trying to cut the United States out of any significant role in Lebanon, Arens is thought by these officials to believe Israel must retain its special relationship with the United States and thus must find accommodations with Washington.

But none of these officials said they expect the change will lead to a quick breakthrough on Lebanon. In fact, they expect the turmoil caused in Israel by the inquiry into the massacre of Palestinian civilians by Christian militia in Israeli-occupied west Beirut will set matters back perhaps by several weeks.

Yet officials said the U.S. hope is that Sharon's diminished status will produce a new power center in the Begin Cabinet focused on Arens and Foreign Minister Yitzhak Shamir, whose position is similar to Arens, and that they gradually will nudge Begin toward coming to terms over Lebanon.

As a result, the officials stressed, while Reagan is not prepared to wait indefinitely, there is increased optimism in the administration that a Lebanon withdrawal can be achieved without a U.S.-Israeli confrontation.

Also yesterday, 51 senators, a majority of the Senate, joined in sponsoring a resolution opposing the sale of advanced U.S. arms to Jordan and calling for Jordan to join peace talks with Israel. The resolution, backed by 37 Democrats and 14 Republicans, was originated by Sens. Edward M. Kennedy (D-Mass.), Rudy Boschwitz (R-Minn.) and John Heinz (R-Pa.).