The vacancy rate for apartments across the US fell sharply in the first quarter and average rent prices rose slightly as the nation's job market continued to improve and many Americans remained either unwilling or unable to purchase a home. Reis's quarterly report, released Wednesday, showed the national apartment vacancy rate dropped to 6.2 percent in the first three months of 2011, from 6.6 percent in 2010's fourth quarter. The drop is the biggest decline in the measure since Reis began tracking it in 1999.Analysts say that the falling vacancy rate can be partially attributed to increased employment, especially for Americans between the ages of 20 and 34. Many of these new additions to the labor force are unable to fork over tens of thousands of dollars required for down payments, thus they become renters. And all these new renters were thrust into a market where supply grew by just 44,184 units.With new construction at a level of just one-fourth what has been typical in recent years, landlords could exploit the supply and demand imbalance by raising rents. Analysts say that conditions are stacking up very favorably for landlords, investors, and managers of multifamily properties, which means bad news for renters.So far, however, rent prices appear to be remaining relatively stable, as Reis reported that the average US rent price increased just 0.5 percent in the first quarter to $991 per month. The group also said rent increases occurred in 75 of the 82 markets it tracks.