On Tuesday, Vietnam’s National Assembly elected a new prime minister. For once, the appointment looks less like a factional compromise than a deliberate bet on competence. Le Minh Hung, born in 1970, is the country’s youngest prime minister since 1955. In a system that often prizes seniority, that alone is striking. More striking still is Hung’s profile: he is not a provincial baron or a deal maker forged in the rough-and-tumble of local politics. Hung is a technocrat with economic training in...