Checked the seatmap again today and the aircraft has now been switched to a Boeing 787-9 with a 2-3-2 configured Premium Economy, even better than the Airbus' 2-4-2 configuration. I think it's unlikely to be switched to a smaller plane size give that my flight is scheduled for a Friday afternoon. I splashed out 270,000 VND (US$10.5) to secure seat 15K.