The March survey, by MarketPoint Realty Advisors in San Diego, shows that the rate is still well below 3%. But at 2.58%, the rate is still at its highest level in five years.

The rise is the vacancy rate is largely due to an increase in the high-end, or luxury, portion of the market--apartment buildings of more than $1,000. Of the 3,035 units reported vacant, just 290 were priced at less than $800 a month, according to the MarketPoint survey.

San Diego apartment rents remained flat for more than a decade, from the mid-1980s until the mid-1990s as a result of the California recession and overbuilding. But in the late 1990s, the area's economy boomed, creating demand for thousands of new units to meet the area's spiraling population.