I had a similar experience on a flight from Saigon to Nha Trang. There were empty seats on the flight but they wouldn't let me purchase new tickets for that flight, forced to take a later flight. My wife and I hung out at the airport shopping mall. Vietnam Airlines gave us money for our inconvenience. I ended up ahead financially after it all, although pulling our luggage across the airport, across a street to the mall was not fun.