THERE MAY be some subject . . . somewhere . . . that has produced more tedious, predictable and repetitive musings over the years than the institution of the vice presidency has. But offhand we can't think of it. The only good thing to be said about this periodic outburst of interest that invariably manifests itself in disinterment of the same quotations from John Nance Garner and aphorisms and perplexities is that it only happens every four years. But it is our sad duty to inform you that it is the time again. Your four years are up.

So let's get started. What is different about the job that Ronald Reagan offered George Bush last night? What, if anything, has been added to the sum total of human knowledge about the vice presidency in the past four years? In Detroit, much of the external aspect of the thing was unchanged. There were the same craven, sickly, sycophantic bows and smiles on the part of certain of the aspirants for the job whenever the name of the Great Man, the Incomparable One reposing up there in the heavily guarded top floor suite was mentioned. There was the same continuous shifting of certainty from one hot prospect to another with the same collection of after-the-fact justifications of why the moment's favorite would be the candidate's choice.

But some of the most durable pieties of the classic conversation on this subject have been affected by the Carter-Mondale years. Like Cabinet-government, which this administration also gave at least a modified try, and which turned out to be a truly terrible idea, so too the living -- as distinct from dead -- vice presidency has been experimented with by the Carter government. By living vice presidency, we mean merely the innovation of actually letting the vice president out of his institutional cage, of trying to substitute useful political and government work for the more traditional and much-mocked idleness associated with the office.

All candidates for president, of course, have pledged to try this out, but only Jimmy Carter has given it a real test. It has sort, more or less, vaguely worked: Mr. Mondale is incomparably more active, more heavily engaged in the staff and policy business of his administration than any vice president we can remember. And he has been spared the familiar vice presidential function (not mentioned explicitly in the Constitution, but derived from things that august document does and doesn't say about the job) of being systematically and unendurably and endlessly tortured by the president and the presidential staff.

So civility and practicality have been proved at least possible and useful in the relationship of president to vice president, and this much we now know. But we also know that the good intentions have had their limits and that Mr. Mondale has been able to function not as a surrogate head of government so much as a top-level trusted aide and that the principal gain from the process has probably been the familiarity he has acquired with the workings of the Carter government. That is another way of saying that deep down what a vice president is there for and what he does is this: stand by to take over if the president dies.

You don't have to disregard the political impact of the candidate on the ticket as a whole to end up believing that this, the backup role, is really in some way the sum total of the office -- what it is about, what encompasses all its importance, truly the only standard against which a candidate should be measured. We know that sounds goody-goody in the political setting of a convention, but it is true. There are more and less interesting and important things to do when you are vice president -- Mr. Mondale has demonstrated that. But there is only one reason for being there and that is to be ready.

If you buy that view, then you will have seen the irony in the discussions that went on concerning a Reagan-Ford ticket. The former president would surely have been "ready" to take over from Mr. Reagan. But the question was whether the job of Ronald Reagan's vice presidient could be made big enough for a man who has already been president. Now, however, the question of whom Mr. Reagan prefers has been resolved. What has not been resolved is the basic anomaly of the role of the American vice presidency.