CYRUS VANCE'S resignation as secretary of state, is true to the logic of his past. As a Pentagon official during the widening of the war in Vietnam, he evidently came to feel that force should be used only in the most dire circumstances and that an official out of sympathy with a major policy should resign no matter the cost to his chief. These conditions seem to have recurred for Mr. Vance in recent months. The administration was moving toward a position on force that he apparently could not stomach; he specifically disapproved of the rescue mission. He was being progressively isolated from the new mainstream of administration policy -- and not just on Iran. The negotiations to which he had earlier made a notable contribution, and to which his problem-solving approach was best suited, had been either wrapped up, stalemated or largely removed from his jurisdiction.

In terms of his personal situation, then, it is understandable that Mr. Vance, who had loyally swallowed many slights in the name of duty, finally resigned. Whether it is understandable in terms of the ethic of public service is another matter. The resignation is a stunning embarrassment to Mr. Carter and deprives him of a trusted counselor in the middle of his most difficult passage. One wonders at the least why Mr. Vance could not have found another time to leave.

There is no denying, however, that Mr. Vance had fallen out of phase with the president. The secretary, and the early Carter, spoke for a benevolent and rationalistic world in which the United States, by accommodating certain ligitimate imperatives of others, would find its proper place. The world to which Mr. Carter, much more than Mr. Vance, has sought to adjust recently is one in which factors of power and pervesity loom large. Mr. Vance was more symbol than spokesman for his point of view; typically, his resignation announcement was brisk and unresponsive. But there was no denying that events had dented his premises. We refer here not just to the normal bureaucratic give-and-take which, to the detriment of his influence, the gentle and aloof Vance found wearying: Soviet tanks played a part too.

The resignation will convey to some anxious foreigners (and Americans) that Jimmy Carter has surrendered to international frustration and the exigencies of the presidential campaign. Though this impression represents, we believe, an exaggeration, Mr. Carter must deal with it. There is no doubt that Secretary Vance did have an important balance-wheel function, and Mr. Carter is now under a special obligation to ensure that counsels of restraint are not banished from his deliberations on Iran and other matters.

One anticipatory response surely was the pledge offered Sunday by Harold Brown and Zbigniew Brzezinski to be more forthcoming in consulting the allies before taking military in consulting the allies before taking military action against Iran. A widening of consultation with Congress is also indicated. Further, it is essential to appoint as secretary of state someone of stature with a commitment to the president's purposes and a capacity for independent judgement. Something more is needed than an administrator or an ideologue or a figure whose personality itself ignites dispute. Mr. Carter needs to show that, though members of his team may be dropping off, he can stay the course.