Vice President Mondale returned yesterday from his round-the-world diplomatic mission, saying the stage is set for a springtime summit meeting of the United States and its allies, probably in London in May.

Welcomed back from his journey by President Carter, who praised him for an "absolutely superb" job, Mondale said he expected plans for the summit to be announced within 10 days.

In an arrival statement at Andrews Air Force Base, where his wife, Joan, greeted him with a hug and Carter with a firm handshake, Mondale said he found "our relations with our friends are on the firmest possible, most hopeful basis."

He brought back to Carter some "personal messages" from several leaders of the seven nations and four international organizations with whom he met, but told reporters they dealt with "sensitive" matters that "I can't talk about."

He is scheduled to make a full report to the President this morning. But yesterday Carter told him, "My only order now is . . . go home and get some rest."

On the last leg of his nine-day, 24,508-mile journey, Mondale told reporters aboard Air Force Two, "I'm a lot less tired than I thought I would be, but somewhere around Thursday, I'm going to fall right on my head."

The Vice President said he thought his journey to Japan and the major European allies - which began less than 72 hours after the new administration took office - was "just perfect" in its timing.

"Having me going immediately was psychologically the right thing to do," he said. "Because Carter is an entirely new international leader for them . . . I found in many cases they were asking questions just to reassure themselves that relations and traditional understandings would be continued . . . In many cases, you could see them relax."

Mondale said that was particularly the case when Carter's own remarks in a Jan. 24 interview caused concern about American nuclear policy.

"It's a good thing I was there at the time," he said, "because I was able to explain that Carter's hopes for complete elimination of nuclear warheads was a long-term process, and that we understood the significance of a nuclear deterrent to Western security interests and that we would work with them very, very closely . . . They asked a few questions (about the Carter interview) that, to me, meant they were wondering what it meant."

While Mondale said his first diplomatic mission was a success, he indicated that he was not likely to repeat it.

"As the secretary of state and the secretary of Defense . . . get into place," he said, "the kind of role I play would be somewhat different. I really went as the first emisary of a brand-new government; that is normally the role played by the Secretary of State, and I believe it will be and should be."

That comment reflected Mondale's general philosophy on his role, which he described as "a general policy adviser" to the President, rather than a competitor with a Cabinet secretary for control of any particular area of operations or policy.

Reporters travelling with the Mondales and Carter by helicopter to the White House heard Mondale promise the President a private assessment of each of the leaders he had met and a written forecast of how they would react to Carter.

Aboard Air Force Two, Mondale acknowledged to reporters that there was "some confusion" about plans for the summit, "because different governments have different notions about what ought to be on the agenda."

Everyone wants to deal with economic questions, he said, but the other countries "are less enthusiastic" than the United States "about going behind the economic issues" to discuss East-West issues, the North-South dialogue or nuclear proliferation.

Some governments - including France and Japan - say they prefer only economic issues to be on the formal agenda, Mondale said, "or, if there are other issues, they should be handled only very informally, over coffee or something."

Mondale said there was "a possibility" the summit would follow the meeting of NATO leaders scheduled for London in mid-May, but the meetings would be separate because the summit will include a broader group of nations.

He said Canada and the European Economic Community would "probably participate" in this summit, even though France and other nations had been skeptical about expanding the group.

In his conversation with reporters, Mondale added little to the guarded statements he had made during the trip on his negotiations on bilateral trade and other issues.

He acknowledged that while "I made a strong plea" for a bigger West German effort to stimulate economic growth, "I don't know just what they're going to finally come up with."

Mondale's relaxed, low-key style was praised in many European newspapers as a welcome contrast to the intense style of former Secretary of State Henry A. Kissinger.

Mondale told reporters he did not think the Europeans were "critical of Kissinger," but he noted that "now that the executive and legislative (branches) are of the same political party, our executive negotiations with them would more likely be fulfilled."

Later, on the helicopter, he told Carter, "They (the Europeans) like Kissinger, but increasingly thought he couldn't deliver on his commitments."