Secretary of State Cyrus Vance can expect to encounter deepseated Syrian suspicion of American connivance in Egypt's go-it-alone diplomacy when he arrives here Tuesday on the most challenging stop of his current Middle East trip.

These suspicions have been sharpened by American backing for the Cairo peace talks, which Syria feels are doomed and has decided to boycott.

The Syrians are persuaded that Israel will make no meaningful concessions in Cairo. The government appears convinced that failure is therefore inevitable and might help slow down Egyptian President Anwar Sadat's headlong diplomatic offensive.

Symptomatic of the suspicious Syrian mood was an official spokesman's snappish warning today that Vance would not be welcome if he is carrying a message to President Hafez Assad from Israeli Prime Mimister Menahem Begin, as a radio station reported.

The report, by Radio Monte Carlo, was denied by the United States within hours. In more normal times the very fact that it was broadcast by the partly French government owned station though one not noted for reliability, would have called for a more prudent reaction.

Assad returned today from a five-day visit to Saudi Arabia and other conservative oil-rich states in the Persian Gulf where he was trying to line up support to oppose Sadat's Cairo conference.

First indications suggest he did not have great success.

The Assad government had questioned American motives even before Sadat's visit to Jerusalem. It had since allowed the government-controlled media to charge that Washington engineered the current split in Arab ranks.

In the past week, "the paranoia index jumped several points," in the words of observer because of official American statements interpreted here as encouraging the Cairo peace talks.

The Cairo conference is condemned here as the first step toward a much feared separate Egyptian-Israeli peace which would leave Syria to bear the full brunt of any Israeli military move.

Vance and Zbigniew Brzezinski, the president's national security adviser, perhaps inadvertently fed such suspicions in recent remarks.

The remarks suggested that the Cairo talks might replace the Genewa formula involving all Middle East Combatant states and that Syria and Jordan might be brought into the Cairo negotiations at a larger stage.

The Assad government shows no signs of entertaining any outcome for the Cairo talks except failure. Implicit in its thinking is Sadat's own fall from power.

The Syrians are expected to argue forcefully in their talks with Vance that the United States has a responsibility to try to slow down Sadat's thrust toward what Damascus sees as undefined goals that are in fact a seperate peace deal.

Neither Syrian officials nor foreign diplomats here are convinced by Vance's protestations that the United States sees itself only in a "supportive" role. The United States is credited with the power to influence both Sadat and Begin.

"Vance better have something positive in hand when he sees Assad," remarked one observer with long Syrian experience, but the Syrians are so pessimistic that no one here expects much to come of the Vance visit.

The most optimistic Syrian scenario foresees a rapid failure of the Cairo talks followed - perhaps in only two or three months - by reconciliation between Egypt and Syria.

Such a reconciliation would be on Syrian terms and implicitly involve Sadat's removal.

Sadat told the West German news magazine Der Spiegel that if he failed he would "offer my resignation and (say) Hafez Assad is right and (he) should take the tiller."

Syria - and many others in the Middle East - are persuaded that even if Egypt and Israel conclude a separate peace, such a unilateral deal would be unworkable.

In a case of failure in Cairo, Syria is believed to favour U.N.Secretary General Kurt Waldheim's proposal for a meeting in New York of all frontline countries.

Israel, delighted to have achieved its generation-old dream of direct negotiations with an Arab country, has rejected Waldheim's idea.

The pace of Middle East events has quickened so much since Sadat's Jerusalem visit that it is hard to tell whether Syria or Egypt is the most isolated of major Arab world powers.

Unlike Sadat, however, Assad has maintained a credible balance between the United States and the Soviet Union which still provides him with weaponarydespite sometimes sharp differences.