WHY ZAIRE?, WE asked on Wednesday. The moment seemed right, since Secretary of State Vance was going up to the Hill that very day to explain why the administration had suddenly rushed two planes full of military gear to that jittery African nation. But, as far as we can see, he didn't really explain it at all. Breaking into the old Foggy Bottom mumble, he merely asserted that if the Katanga soldiers who've crossed from Angola into Zaire succeeded in threatening the copper mines, Zaire's stability would be imperiled. He provided no serious rationale at all for why the stability of Zaire is of such importance to the United States that the administration should ignore its own professed concern for human rights (Zaire is a real loser in that department), identify itself with one of Africa's more corrupt and ineffective governments, and accept an involvement in a dispute with such a raw, ragged, runny look to it. The stability of Zaire: is that what the Carter administration's brave new African policy means? Shades of John Foster Dulles, Dean Rusk, Henry Kissinger and the whole balance-of-power gang.

If Secretary Vance's performance on Zaire was dismal, however, that of the International Relations Committee was appalling. Where are the no's of yesterday? What happened to those vigilant tigers supposedly so ready to ensure that executive power is wisely and well used? Lapping cream out of the administration's saucer, that's what. Aside from some sharp questions by Rep. Don Bonker (D-Wash.) - questions which Mr. Vance slipped by - the rest of the committee simply ignored its responsibility to check on what is after all the Carter administration's first brush with a foreign firelight. It was enough to make you wonder where Congress had been for the past 10 or 20 years.

No, we do not think that Zaire is "another Vietnam." But we do think that the administration has reacted rashly and reflexively to an old client's dubious appeal for emergency aid. Is Angola, in helping a dissident group launch forays into Zaire, doing anything that Zaire has not done recently, and may even still be doing, to Angola? Cannot a way be sought - in particular, by Zaire's European or other American friends - to tame by diplomatic means the running dispute between Angola and Zaire? Are the United States' accumulated obligations to Zaire so overwhelming that the administration could not briefly pause to see what else might be done? At the least, could not the Secretary of State have deigned to spell out the considerations which seemed to him to make it necessary to fly to President Mobutu's aid? This is the Carter administration, isn't it?