Once again Secretary of State Cyrus Vance is on the road, and once again be is traveling with an empty brief case. Even if the administration had been able to make up its collective wavering mind over what to do about the People's Republic of China and the island of Taiwan, Vance in Peking would be powerless to do more than make a gesture of goodwill toward an ultimate solution.

To have taken any positive step in the direction of de-recognizing Taiwan and granting full diplomatic recognition to Peking would have put in further jeopardy the administration's immediate diplomatic objective. That is ratification of the Panama Canal treaty.

The same senators determined to block the treaty are opposed to any change in the status of Taiwan. To make any more at this time to change the relationship with Taiwan, which has benefited so handsomely by American investments and American trade, would harden the opposition to the Panama agreement, and that opposition may in any event be sufficient to prevent a two-thirds vote of ratification.

Then it is fair to ask why Vance was sent on what is described officially as an "exploratory" mission. Because the trip was planned long in adv ance, the dilemma of Panama may not have been foreseen. But that is another way of saying that the foreign-policy planning of the administration is Iamentably weak.

It is unfair to Vance to send him on a mission as futile as his recent tour of the Middle East. That tour ended with merely a promise of more talks in this country with the foreign ministers of Israel and the Arab states in the fall. It came after President Carter had met in Minister Menachem Begin and a report of their harmonious exchange.

That was quickly deflated with Begin's announcement of new settlements to be established on the West Bank of the Jordan River. Vance's efforts to get a compromise agreement for a peace conference in Geneva came to nothing.

Carter had insisted before his election that his Secretary of State would not be flying all over the world, that the Carter diplomacy would owe nothing to the Kissinger shuttles of the past. Vance has already traveled as far in his first six months as did Kissinger in the time he was working out the Sinai agreement between Israel and Egypt.

The Secretary of State keeps a low profile. He has none of the spectacular Kissinger qualities of performance and persuasion. It is perhaps his misfortune that he had to follow such a conspicuous performer.

But able and dedicted, he has thus far accepted as the good soldier assignments that promised to end in futility. And this, it should be added, inspite of a health problem. Due no doubt to so much travel in planes with stopovers all too brief, Vance has had a recurrence of severe back trouble.

Vance in Peking will find the People's Republic fretting over the delay in normalizing relations with the United States as called for in the communique signed in 1972 at the end of Richard Nixon's visit. For Peking, normalization means full diplomatic recognition and an end of the American military presence on Taiwan. They have mad it clear they will not be satisfied with anything short of that, and for all the good will of the Vance mission, relations with the most populous nation in the world can be expected to deteriorate.

Important voices have been calling for a shift from Taiwan to Peking. That was the object of a speech by Sen. Edward M. Kennedy (D-Mass.). One of the most vocal - and he should be one of the most influential - spokesmen for normalization is Thomas L. Gates, who was for a year head of the U.S. liaison mission in Peking.

A former Secretary of Defense and former chairman of Morgan Guaranty, Gates has lost no opportunity since his return in the spring to advocate a break with Taiwan and full recognition of the People's Republic. Gates saw for himself, in a year of great trouble that included the deaths of Mao Tse-tung and Chou En-lai and the Tangshan earthquake, both the needs and the strengths of the 850.000,000.

The Taiwan lobby is a powerfula force, with the economic lever apotent one. Some members of Congress, even those like Rep. John Brademas, the majority whip, who have visited mainland China, are likely to say. "What's in it for us?" The answer is surely not an immediate gain in trade with Taiwn. But it is a steady and expanding relationship with a power certain to shape the future destiny of Asia - and the world.