Though the stock market may be hitting two-year highs, there’s not much to celebrate in the housing market. Nearly 11 percent of American houses were vacant in the final quarter of 2010, Census data show, adding up to 18.4 million empty homes across the country. That’s despite real-estate prices that continue to plummet, creating great opportunities for the relatively few Americans who are willing to buy. Renting, conversely, has skyrocketed: 38 million housing units were rented in the past year, up by over 1 million from the year before.