It's a renter's dream and a landlord's nightmare: a market with plenty of vacant apartments, a situation that can drive down the cost of rent. Apartment vacancies outside Denver hit their highest level in four years during the second quarter, rising to 9.1 percent, according to a report released yesterday by the Colorado Division of Housing (cited by The Denver Post). That's about the same rate as inside Denver. Factors driving the trend include too many new units on the market and competition from foreclosures. But the No. 1 cause is unemployment, which has forced many people to move in with a relative or to get a roommate. In Fort Collins, the home of Colorado State University students and a lot of shared housing, vacancies are the highest. The city's rate of 9.9 percent edged out Colorado Springs' rate of 9.8 percent, notes the Northern Colorado Business Report. Far west of Denver, Grand Junction has the lowest rate of any metro rental market, but vacancies there increased from 4 percent in the first quarter to 4.5 in the second, notes the Sentinel.