Just in time for summer, a rivalry between vacation-rental companies is brewing. Last month, Vrbo crowned itself the "hotter, cooler, friendlier" platform with a billboard teasing rival Airbnb.

Vacation rental company Vrbo took a swing at its main competitor, Airbnb, this month with a provocative billboard campaign. It may not have worked. "Think of us as Airbnb's hotter, cooler ...