At what number should a city's vacancy rate be a concern?

[Derek Boettcher]

I'm a new investor living in the greater Kansas City area. I've been doing research on population growth and property demographics in the area and I noticed an interesting statistic. Apparently the vacancy rate of properties is about 15%. Kansas City has started seeing a boom in population growth over the past couple of years, and with about 80-90,000 properties being used for rentals alone. 

To my question: is 15% vacancy a concerning number? What does this rate look like in your markets? At what point does vacancy percentages become a concern or is it really only one part of the puzzle? Other than strong marketing, what are ways to combat a high vacancy rate in your area? Is there a plus side to higher vacancy rates?