ELMHURST, Ill.—Victoria Vasconcellos, the petite founder of an Internet retailer in this Chicago suburb, is in the thick of a regulatory battle that could affect millions of American cigarette smokers.

Ms. Vasconcellos imports electronic cigarettes from a Chinese manufacturer and sells them on her website, Cignot.com, to 14,000 customers. The 48-year-old is part of a growing legion of e-cigarette purveyors who are defying the Food and Drug Administration, which contends the nascent nicotine products are drug devices that require...