That's the point they're making though.
South Bend Indiana currently has a vacancy rate of 12%.
NYC's is currently 3.75% A "full" vacancy rate is about 5%, once you get past that, rents start increasing because housing demand has outpaced supply. And NYC's has been under 5% every year except for two in the wake of the recession, which for obvious reasons are an outlier. There aren't enough houses there.
Housing as speculative investment is indeed a big part of the problem, but "vacant houses" aren't really the reason it's a problem in most areas- it's houses that could be built, but aren't.