A young, tech-savvy population short on cars but big on smartphones is driving double-digit growth for ride-hailing apps Uber and GrabTaxi in Vietnam, and inspiring some executives to dream of replacing private car ownership altogether. Uber launched its service in Vietnam last year and says Ho Chi Minh City and Hanoi already have the highest average numbers of trips per user out of the 300 cities in which it operates, double the rate in New…