Final assembly means that most of the smaller components that can be installed have been installed, and all that's left is to join the large body sections such as the flight deck, fuselage, wings, engines, and landing gear. This process can take a month or more to complete.
VNA is due to receive the 787-9 (B789) in late May, with the first scheduled long-haul flight scheduled for Hanoi (HAN) to London-Heathrow (LHR) on June 29. It will be painted in VNA's updated livery that has already appeared on a number of Boeing 777-200s (B772) after repainting, and on the new Airbus A350-900 (A359) that emerged from the paintshop in Toulouse recently.
(H/T: All Things 787)