Nearly a decade ago, one of China’s largest textile exporters concocted a plan to hedge against trade barriers championed by US President Donald Trump during his first term. Its owners were convinced that moving their primary production base from China’s east coast to Vietnam would be a permanent workaround – bypassing complex supply-chain pitfalls by hunkering down in a tariff lowland and continuing to stock Walmart shelves with socks and towels. Trying to get the jump on Trump in 2017, from...