Arrival to SGN was relatively painless.
Travelled economy which meant a 20min queue to check in at Heathrow (premium economy and business both had no queues, but then that's likely due to the number of travellers in each). At Heathrow they do a document check for insurance, vaccination status and PCR (I think it's a five or six point checklist they verify) and this is what takes a bit of time, prior to you getting to the check in agent - essentially a queue to a queue.
Nothing different on the plane... Normal meals and drinks service.
On arrival, it's somewhat a scene from a plague movie with all the PPEd staff - but pretty seamless. My only comment would be to definitely find where the ATM is prior to beginning to walk from the plane - we didn't think to look until after we had our baggage, at which point the only ATMs were cordoned off and we weren't allowed to use one (didn't really matter in the end).
Immigration took a little time, mainly because we got a slow queue and because again, they want to check lots of documents.
You activate the tracetogether app post immigration, enter baggage and then after baggage have the PCR test. They use little stickers to show that you are in a VTL lane from immigration.
Dedicated taxi rank after the PCR test, the vaccinated status took 2-3 hours to update on the app after immigration, and our PCR test took 5.5 hours.
All in all, fairly painfree - though the A4 binder of documents has made a comeback. If possible I would say print and staple your documents by traveller, and if possible add little bookmarks too 😂. I think the main fear is a positive PCR on arrival, but maybe avoid public transport to the airport and people generally in the fee days leading up, and the PCR on departure should cover before that.
Still rule of two here for everything; but that is OK for us.