U.S. Secretary of State Cyrus Vance has postponed his visit here by 24 hours at Syrian request reportedly to allow President Hafez Assad to confer with a high-level Soviet emissary, expected Sunday, it was learned today.

There was no official confirmation of the Soviet mission or of the emissary's identity.

But Vance will now arrive here Tuesday - after his visit to Jordan rather than before going to Jordan, as originally planned.

The timing of the reported Soviet visit would underline Syrian determination to resist Vance's blandishments aimed at persuading Syria and Jordan to attend next week's Cairo talks and somehow preventing a further split in the Arab world.

So far only the United States and the United Nations have named representatives for the Cairo talks involving direct Egyptian-Israeli negotiations.

The Soviets - along with all other Arab countries - have declined the invitation issued after President Anwar Sadat's visit to Jerusalem.

Observers suggested Assad was trying to strike a balance between the superpowers as an object lesson to Sadat whom he has criticized for his one-sided pro-American policy.

Thus Assad is said to have welcomed the Vance visit because the United States remains on record as favoring a comprehensive Mideast settlement - even if he fears a separate Egyptian-Israeli peace.

In keeping with his prudent policy of playing off the superpower, soon after Sadat's visit to Jerusalem Assad dispatched Foreign Minister Abdel Halim Khaddam to Moscow where he was reportedly "well received" by Soviet President Leonid Brezhnev himself.

However, the Syrians are well aware that increasingly the Kremlin has called for an allout condemnation of Sadat's go-it-alone diplomacy in intrasigent terms scarcely distinguishiable from those of Syria's arch-enemy, Iraq.

Assad was meanwhile reported winding up a three day visit to Saudi Arabia and other Persian Gulf states.

Soviet propaganda has accused the United States of having plotted the direct Egyptian-Israeli contacts and sabotaged the overall Mideast peace talks planned for Geneva.

The Soviets have been showing signs of trying t cover their flanks in the face of American support for bilateral Egyptian-Israeli negotiations by endorsing the hardline Iraqi rejections of any peace negotiations.

Nor, despite Syrian dependency on Soviet arms, has Syria forgotten the Kremlin's criticism of its role in ending the Lebanese civil war.

Assad himself has stressed that despite its sharp differences with Sadat there was "no divorce" between Egypt and Syria.

Still, the Syrian foreign ministry felt obliged to issue a denial that Jordanian King Hussein's midweek visit here was a mediation effort between Syria and Egypt.

If nothing else the denial deepened the impression that Syria now feels that there will be a long period of diplomatic stagnation concerning an overall settlement.

Any movement is likely to come in the direct Egyptian-Iraeli talks, it is thought here. Even putting together a larger forum - somehow including Jordan, Syria and perhaps the Palestinians, now is thought to be far in the future, according to observers here.

Assad's trip to Riyadh - and that of King Hussein - underlined the growing role of Saudi Arabia which helps finance Egypt, Jordan and Syria.

Angered with Sadat for not consulting with them before his Jerusalem visit - but not about to drop him in favor of a very possibly less friendly successor - the Saudis are sooner or later expected to try to patch up this latest inter-Arab quarrel.

Only two years ago, the Arabs were split in a similar fashion by then Secretary of State Henry Kissinger's success in negotiating the Sinai II military pullback deal.

Iraq then and now remained true to its odd-man-out total intransigence. Syria, if anything, was more vehement in its accusations of Sadat's treason in leaving it at the mercy of the Israeli military machine.

And Sadat tested the "Egypt first" line which has proved so enormously successful with his impoverished, war-weary people.

Then it took 13 months - from September 1975 to October 1976 - for the Saudis to persuade Egyptians and Syrians to sink their differences at the Riyadh summit which also was instrumental in ending the Lebanese civil war.

Militating against Vance's attempt to win over Assad is the knowledge that the visit precedes the opening of the Cairo conference.

Syria would be foolish to make any commitmet before knowing what, if anything, the Israeli are willing to provide Sadat in return for what his critics claims was his unconditional surrender in Jerusalem.

Assad is widely credited with criticizing Sadat for playing cards he should have kept for subsequent bargaining.

Barring an unforeseen miracle, Syrian officials appear convinced that Israel will offer Sadat little more than a separate deal on the Sinai - scarcely the kind of overall settlement he claims he is seeking in the name of all the confrontation states and the Palestinians.