What’s next for Vuori after a $5.5 billion valuation? A three-way stretch

On a sunny Friday afternoon in June, I walk down London’s affluent King’s Road to a two-block stretch that features four athleisure stores. At Alo, racks of sleek matching sets seem designed for women who post their Pilates workouts on Instagram. At Nike, posters feature life-size photos of its sponsored athletes at the World Cup and Wimbledon. At Lululemon, under a heavily sloganed gray wall (“Friends are more important than money”), there’s a giant sales rack featuring a mishmash of athlet...