PREMATURE AS IT may be to pronounce the place the Soviet Union's Vietnam, it is undeniably satisfying to observe the troubles the Kremlin is having in settling down Afghanistan. That remote and backward land was jostled out of decades of internationally accepted neutrality a year ago by a coup that installed a Communist faction backed directly by Moscow. Whether that coup came in reaction to an impulsive effort by the shah to bring Afghanistan into the orbit of Iran, as some claim, or in fulfillment of a Kremlin design, is arguable. What is evident, however, is that Moscow, finding its client set up in Kabul, pitched in to help it consolidate its power and has been unable so far to succeed.

There seems to be sharp fighting going on, involving religious, ethnic and conservative elements offended by the new regime's Marxist and centralizing bent. Tribes and political groups that live close enough to the Pakistani and Iranian borders to get help from the other side have been matched against an army equipped and advised by Moscow but evidently not fully loyal to President Nur Mohammed Taraki. Some of the several thousand Soviet advisers have been caught up in combat and a number of them killed. The bottom is not necessarily about to fall out on the Russians but the likely prospect is a continuing drain.

The spectacle of Moscow's embarrassment furnishes a useful antidote to the notion that blossomed in this country a year or so ago to the effect that the Russians were coming across a broad Islamic "arc of crisis" stretching from Afghanistan to the Atlantic. Now it turns out - and not just at this one point - that the arc poses its problems to the Soviet Union and the United States alike. The new Islamic government in Iran, which some in the West marked off as an easy target for the Russians, is putting some distance between itself and Moscow. Some perspective in assessing the threat that local change means to the United States can now perhaps be regained.

Something needs to be kept in mind about Afghanistan. Like many Third World places, it does not have to be in the Western column for American interests to be served. The leading American interest there is stability: The place should not be regarded as up for East-West grabs. So Moscow's discomfort should not put too much of a glint in the eye of American strategists. Before last year's coup, the long - standing balance of the region - a balance long predating the Cold War - seemed on the way to being upset by Iran. The coup tipped Afghanistan the other way. The balance needs to be restored. The Soviets may yet find it to their relief and advantage to have that happen.