Just to add another data point here, just arrived from BKK transferring to domestic flight in Business Class first time at SGN. As described by others it seemed very busy and chaotic but organised chaos is fair description but it worked very well from my point of view.

I had been a little apprehensive having read so many horror stories but just before immigration there was a station setup for those transferring one member of staff from VN put stickers on us and another walked us to a nearby barrier where it was opened and we were directed through the empty wheel chair lane and was stamped in 30 secs max. Had to go and collect bags but they were out in 15 min or so, straight through customs and there was a desk setup with a member of VN collecting bags and marking names off a list.

Originally thought it might be tight but arrived to the domestic lounge within 35 min of touching down. I will say VN staff on the ground were great guiding us through which is obviously a big factor in it being so fast and smooth.