Next door to Israel, two or three minutes away by supersonic jet, is a witch's brew, double, double toil and trouble. The many-sided conflict in the half-ruined state of Lebanon has repeatedly threatened to spill over, and in fact rockets from the militant Palestinians have struck Israeli communities.

Syrian troops occupy perhaps half the country. All except the militant PLO agree that they have been a force for peace, putting restraints on the war between Moslem and Christian combatants.

While Vance was in Saudi Arabia, he received an abrupt telegram from Hafez Assad postponing the secretary's visit to Damascus. With the dogged patience he has shown on a tour producting little tangible results, Vance waited 24 hours before going to Syria. Of his four-hour meeting with Assad, little could be said other then that he got no encouragement.

Awsad, who has put a brake on the fiercely conflicting forces in the region, is an object lesson. Intelligence reports, accepted by top U.S. policymakers report that he is suffering from an illness likely to prove fatal in six to nine months.

His loss would leave a large hole in the fragile structure of no peace, no outright war prevailing in Lebanon today. He could be replaced by a wild man with no restrain whatsoever, like his foreign minister. But Khaddam Abdal-halim, who is also deputy prime minister, has no real power base of his own, and in the ensuing uncertainly the leadership would go to a successor with clear lines to several factions in the Syrian military.

The object lesson is the tenuous nature of peace in the Middle East, as it depends on a few individuals. With his courage, his integrity, he is an indispensable man.

Whether the Israeli government understands what he means to the peace equation is doubtful, judging by the reluctant attitude of Prime Minister Menachem Begin in the aftermath of the Camp David talks. A successor to Sadat would hardly be willing, or able, to make the concessions that he made at the summit.

Toward the end of his address to Congress, President Carter put in a few remarks about Lebanon, calling for joining in efforts to bring an end to the conflict and suffering there. Those who look candidly at the Lebanese situation believe the United States can do little beyond the modest military assistance now going to Beirut.

That is in striking contrast with the action of President Eisenhower in 1958, when American power and influence were at a peak. At his orders American Marines landed on the beaches at Beirut to quiet a situation that might have developed into civil strife. Their pretence resolved the relatively simple dispute, and then they withdrew.

A measure of the deterioration is how infinitely more complex is the tangle in Lebanon today. The Maronite Christians fighting the Moslems are themselves divided into two warring factions. While those in one faction are not beyond occasional sniping at Syrians, the leader of the Falange Party - opinion the second faction - recently gave his opinion that it would be folly to ask the Syrians to leave.

Lebanon in 1978 is receiving $25 million in military sales, with an additional $75 million now before Congress. Americans with interests in Beirut still intact argue the need for a small military assistance group of perhaps 100 to 200 officers and men to help in training Lebanese government forces in the use of the new weapons.

But those dealing daily with the situation believe that is unncessary. They argue that with more determination President Elias Sarkis could forge and effective fighting force that could conceivably bring an end to the factional conflict. The Lebanese do not need money. Estimates here put their reserves at $3 billion, and many Lebanese descendants of ancient Phoenician traders, still have considerable wealth.

To have added Damascus to Vance's whirlwind Mideast schedule seemed at the time like cruel and inhuman punishment. Assad was assembling the leading Arab rejectionists to swear their determination to wreck the Camp David agreements and wreak vengeance on President Carter. For wild men like Yasser Arafat it was a tribal dance of vengeance.

Their violent vocalism to one side, the United Nations contingent on the Lebanon-Israeli border is holding firm with the militant PLO pushed out of range of lobbing missiles into Israel Vance's talk with Assad hardly makes a bad situation worse. It might even, at least in understanding, mean a slight improvement.