"Sales are going really well, and getting better every month," said Jen Osmun, who started the business with her husband, a former plumber, after he was injured in an accident. Most customers at Grassy Plain Vape & Smoke buy electronic cigarettes to help them quit smoking tobacco, but a growing number are referred by medical marijuana commissaries in the neighborhood - the nearest is about five miles away.

The Osmuns' experience is becoming more common as the number of U.S. vape shops soars and shop owners seek to capitalize not only on the vaping trend, but on the more widespread, and legal, use of medical marijuana.

Since 2008, the number of U.S. vape shops has grown to about 8,500, and the sale of electronic cigarettes and supplies climbed to $3.5 billion, according to Wells Fargo Securities analyst Bonnie Herzog. She expects U.S. use of e-cigarettes and vaporizers to overtake combustible cigarettes in 10 years.