IN THE vice presidential debate, so far as a mastery of the facts is concerned, who won? Or, more precisely, who lost in the sense of erring most gravely on the facts? It was a split decision. Rep. Ferraro was weaker than Vice President Bush on foreign policy and national security material. She was, on the other hand, highly factual on domestic questions -- whereas he was anything but.

First for the domestic side. "When we came into office," declared Mr. Bush, "why, inflation was 12 1/2 percent, interest rates wiping out every single American were 21 1/2, if you can believe it. Productivity was down. Savings was down. There was despair. . . . And this president has turned it around." If you can believe it, the vice president said. Well, can you?

Official statistics suggest not. Mr. Bush is right that inflation averaged about 12 1/2 percent in 1980 and that reducing the rate of price increases has been the major economic accomplishment of this administration. But the 21 1/2 interest rate, which Mr. Bush cited repeatedly, is very misleading.

The prime rate -- which reflects essentially the cost of funds to banks -- did touch 21 1/2 percent in January 1981. But the rates the average individual or corporation actually paid were much lower. For example, interest rates on new mortgages and on AAA corporate bonds were both slightly above 13 percent at the start of 1981. Rep. Ferraro was right when she pointed out that during the Reagan administration interest rates for home buyers averaged about 14.5 percent -- well above the supposedly intolerable 10.5 percent average for the Carter years.

When it comes to "wiping out Americans," Mr. Bush seems to have forgotten that the deep recession of the Reagan years set post-Depression records. And Rep. Ferraro is also right that the 10 million jobs added during the Carter administration is a much more impressive record than the 6 million added during the recent recovery after earlier losses.

Mr. Bush is also wrong about savings. The personal savings rate hovered around 6 percent in the Carter years, rose a bit in 1981 and then -- despite all those supply-side incentives -- dropped below 5 percent in 1983. Now it's back to the same old 6 percent. After a drop during the 1982 recession, productivity is up over the generally dismal record of the late '70s. So far the improvement has been average for a recovery from so deep a recession.

Mr. Bush asked "the experts to go to the books" to check his assertions that "Human resource spending is way, way up. Aid for dependent children is up. Immunization programs are up . . . Spending for food stamps is way, way up under the Reagan administration." We asked the experts too. They told us that Mr. Bush is wrong on every count. Perhaps that's because he didn't take account of the fact that the poor, like everyone else, are paying higher prices than four years ago and that, thanks in part to administration policies, there are more poor people now.

Adjusting for inflation shows that these programs have either declined over the four years or, at best, held even. Even the tiny immunization program -- which the administration sought unsuccessfully to cut in its first two budgets -- has only kept pace with general inflation, while vaccine costs have soared. Adjusting for increases in the number of poor people as well shows that both Aid to Families with Dependent Children and food stamps have declined by about 14 percent. If the administration had gotten all the budget cuts it asked for, the cuts would have been more than twice as great.