I was browsing my current issue of
Travel & Leisure and happened on an article about luxury train service from Da Nang to Quy Nhon, gateway city for Zannier Bai San Ho where we have a booking. It's a car that is attached to the regular daily train service between those cities.
https://www.thevietagetrain.com/
https://thereshegoesagain.org/the-vi...train-vietnam/

It sounds really pleasant and I have now booked it. Prices jump to $400 per person in December.