I use the excellent HKG Airport "My Flight" app.

It is almost real-time linked to the notice boards in the airport terminal and at the gates.

When they start boarding, a popup tells me within 30 seconds that the status is now boarding. Ditto for final call, gate closed, delays etc.

It is faster with delay reports and ETD/ETA updates than TripIt or NotiFly.

You pop in your inbound and outbound and let it notify you. Obviously in the case of sitting at SGN it will only show you whats up with the HKG-SGN and then it will show you ETA updates for your SGN-HKG if you set it up that way.