DETROIT — Flint is the most vacant city in the nation, according to a new report. Detroit is a close second. The report by RealtyTrack, which provides housing data nationwide, says 7.5 percent of homes in Flint are vacant. Detroit is not far behind with a 5.3 percent vacancy rate.But the two Michigan cities are exceptions to the national trend, where vacancy rates are low.”With several notable exceptions, the challenge facing most U.S. real estate markets is not too many vacant homes, but too few,” said Daren Blomquist, vice president at RealtyTrack.The Motor City and Vehicle City, once thriving industrial hubs, are suffering from outsourcing and the decline of the auto-industry.About 160,000 people lived in Flint in 1980. Fewer than 100,000 reside there today.Detroit’s population peaked at 1.8 million in 1950, compared to fewer than 700,000 people today.