Because rental vacancies are at an all-time low, housing experts and government officials have been saying that a rental housing crisis is at hand.

Although monthly rents are going up, they're not rising fast enough for apartment owners to make ends meet, and owners are opting out by converting to condonimiums or abandoning their buildings altogether, the experts have claimed.

But recently some economists have begun to think that the crisis may be largely overstated. In fact, some suspect that there may be an oversupply of rental units that has resulted as more moderate-income families buy homes.

Kevin Villani, acting chief economist for the Department of Housing and Urban Development, takes this view in a forthcoming report to Congress and in a book to be published in June by the Urban Institute.

Two years ago, the General Accounting Office reported that "millions of Americans cannot afford home ownership and cannot find affordable rental housing." The agency warned that "immediate national attention is necessary if an adequate supply of affordable rental housing is to be available."

Villani disagrees. "There's a surplus of rental housing," he asserted in a recent interview. "People want to get out, and owners [want] to convert." a

The economist argues that inflation and tax policy make homeownership more advantageous for middle-income persons than renting. That's why the percentage of homeowners rose from 61 percent in 1970 to 67 percent in 1978, he contends. This shift has contributed to a surplus of units and, in turn, to declining rental prices, he says.

Villani bases his position on research showing that rents have not kept up with inflation. "There's a myth that renters are worse off, that rent is taking an increasing share of their income," Villani said. "That's not true."

In fact, he contends, tenants are spending a smaller portion of their incomes for rent. Villani argues that if there were more demand, rents would be higher. As it is, owners must adjust to the market through abandonment, condo conversion or lower rents, he said.

Low vacancy rates would seem to challenge the theory that there is an oversupply of rental housing. But Villani contends that those statistics measure the number of habitable units. When housing stock is excessive, owners depreciate their buildings faster and pull them out of the stock quicker, he said.

Demand for rental housing may begin to pick up, Villani said. High mortgage rates, increasing housing prices and an end to the fixed-rate mortgage is making home ownership a less desirable investment, Villani predicted.

Some families may choose to remain renters at least for a time, he said. Although this could push rents up, it would take quite an increase to encourage further production, the HUD economist argued.