I’ll add a recent data point about departing SGN with an international VN business class ticket on VN metal. Was able to get into an unmarked fast track immigration lane between the wheelchair and APEC lanes. The whole section was roped off and an airport worker would let people in after showing credentials (in my case my business class boarding pass).

I asked the VN check-in agent about immigration fast track and they told me “lane 4”. Since the lanes aren’t numbered, I would have had no idea where to go if not for this thread. When I was in line I counted and sure enough, it was the 4th window from the right.

The check-in agent did not offer any of this info, I had to ask.

All in all, it took me about 5-10 mins to clear immigration and security on a Saturday afternoon.