Back in 2009, something odd began to happen. The economy was so bad that suddenly landlords in some locations were forced to begin slashing rents to keep their tenants. This, of course, delighted renters. But their euphoria was short-lived: we're now seeing rent prices skyrocket, up 6.7% nationwide since June 2010. One reason why: vacancies have been declining.

Here's a chart from the Mortgage Bankers' Association's report on commercial/multifamily real estate for the first quarter:

Focus here on the blue line for apartments. At 6.2%, the vacancy rate in the first quarter matched the third quarter of 2008. At this level, you can see that it's already back to the normal range of where apartment vacancies were over five years prior to the financial crisis.

Vacancies may continue to decline. Although buying and renting are generally viewed as substitutes in economics, a few changes in the real estate market might skew their usual relationship over the next few years. For starters, more Americans are having trouble qualifying to buy a home, so they will be forced to rent. Construction levels have also been very, very low over the past few years, which indicates that a rental shortage could soon be upon us if vacant houses aren't converted into rental properties in a timely fashion.

Unfortunately for tenants, these trends may continue to push their rent prices up. Eventually, the rent-vs-buy ratio will become too high to ignore, banks will loosen their standards, and home purchases will rise. But that could take a couple of years. Until then, expect the rental market to stay hot, and pricey.